What is equality?
The word is so widely used, and is drawn in such positive light, yet so undefined.
Biologically humans differ to such a degree where we use categories to distinguish groups (superficially perceivable patterns): Race and ethnicity. Another major differences occur physically and psychologically between the two genders. We also differ genetically and culturally, and regardless what the source of the difference is, people are born with different personalities, capabilities, opportunities, bodies and behaviors. This goes even at the level of siblings - further escalating as genetic distance grows. There is also a factor of randomness (unpredictability), even arbitrariness (mutations).
Intelligence, productivity, creativity, interests, ambition, persistence, strength, health, environment (and its influences) - these are all factors which differ and have significant influence on the resulting personality. Not only that, somebody who is intelligent and hard working is likely to become much more wealthier (especially in a relatively free market) than somebody who is not (see: IQ and income). Somebody can be born with a great voice or are inherently gifted, which allows them to become successful musicians. People who are higher also have much better chances to be successful in Basketball - up to a point where people below a certain height may never be able to become one of the best.
What is equal? Well, for one we are all the same species and our reproduction is compatible. It appears, that on this scope the differences are just a question of detail, but the details are exactly which make every human not only unique, but even extraordinarily different, sometimes even seemingly alien. One person can be an emperor, ruling over millions of people, and another one can be a slave, devoid of any power and self-determination. One person can be a billionaire mastermind businessman, trading effectively world wide with millions of people, the other could be just a low-level drug dealer.
Now, there is a counter-notion to that, which is the ancient idea of Tabula Rasa, which is however also inhabited by (neo-)Marxists. It assumes that humans are - at least intellectually and spiritually born "equal". Also they are born "perfect", and subtractively deprived of it as society/humankind is corrupt and flawed. So in this sense the state of equality is also equated with the idea of perfection.
Is the shout for equality an expression of objective truth - is it just a clarification, a friendly reminder that we are "equal" to those who may have forgotten it? Or is it a call to arms - against those who claim that inequality indeed exists, which is what makes us diverse in the first place in uncountable regards? Or even against those who grew beyond what people consider "equal enough to count as the same"?
Given that for 1-2 centuries the notion of "equality" is propagated, often under the banner of Democracy and Socialism, the latter may be the case - a call to arms, an stern reminder. It's a warning to those standing above or apart from the bulk of society - from the great mass of averageness. It's a warning to the individual to subjugate itself into the collective.
In this sense equality is a counter idea to the individual, and a tool of collectivism and collectivists to pursue its agendas.
Now, if the idea of equality is long enough propagated, people start to believe it in an abstract, unclear way (which means it is something like a superstition). When people happen to vastly outpace them in some way, even in magnitudes they can't even understand, it means to them that this person must do something unethical in addition to being somewhat better. After all, they are supposed to be "equal"... so where comes that difference from? A common conclusion is that something wrong and unfair must occur - that person must be unethical or lucky.
The Marxist's answer was collective exploitation:
Starting with the conjectural premise that social change occurs because of the struggle between different classes within society who are under contradiction against each other, a Marxist would conclude that capitalism exploits and oppresses the proletariat, therefore capitalism will inevitably lead to a proletarian revolution.
In this case, that other way more successful person is declared to be part of a different collective, and that collective is out to exploit yours. But it can't help itself, because all collectives (nations, races, ethnics, genders, etc) are in continuous struggle with each other anyway. Also, given the materialist views, the difference in competence is only due to environmental factors (parents, wealth, opportunities - "being born rich").
That way the equality principle is restored: Every cause for inequality is due to unethical or unfair factors. So in the case of "being born rich", it is just a continuous perpetuation of exploitative behavior overarching many generations.
Is equality good?
Depends on how you define and apply it. Both concepts of equality and inequality have been used for good and evil (simply said). Inequality has once justified slavery, feudalism and discrimination, but it also justifies individual liberties and the promotion of great minds (instead of ostracization). Equality justifies genocide*, but also the idea of every soul having an inherent, unquestionable, equal value, thus murder and crimes being unacceptable no matter the "actual value" of the victim or the perpetrator.
*Justifies genocide (like putting people into gulags to work and starve to death) in the sense that if you assume "ought-to-be"-equality, then "is"-inequality is an indicator of some corruption or flaw, justifying radical actions taken to combat inequality by all means necessary.
Why is equality considered good?
Because the dominating definition today is emanating from Marxists and neo-Marxists, who are occupying the political left all throughout Western Civilization. And as of Europe, they are the dominating political force (as self-declared "Social Democrats", which is a moderate version of these ideologies). Also the right side of the political spectrum, which includes Christians, do not oppose the idea of equality either - they just have a different interpretation. Democracy also promotes the idea of equality (for example in regards to voting and how jurisprudence works).
It appears like the idea of equality is only contested in details (equality of opportunity vs equality of outcome), however that alone makes a lot of difference. The difference can be a relatively free society vs a totalitarian society. So even libertarians (Anarchists/Minarchists) tend to argue that what matters is equality before the law (which is the same as the actual rule of law also for those in power) - instead of contesting the idea of "equality" in itself (how could they? It's a broad, undefined term used mostly to virtue signal - devoid of meaning unless specified).
Most ideologies and philosophies have their own ideas of equality, even though they may disagree with each other. Christians and conservatives are not averse of the concept of equality of opportunity, while for (neo-)Marxists it is just a stepping stone towards equality of outcome.