It's because if whatever follows constructions like it is nice, it is good, it is bad et cetera is a verb, it must either be in its infinitive form (infinitives are formed by preceding the bare infinitive of a verb with the particle to) or in its gerund form (you can even refer to it as a present participle or more generically as an "ing" form since in this particular case it really makes no practical difference what you call it). It's an either/or situation and there is nothing in between. That's why. hearing is not the infinitive of hear. It's the gerund form of hear. The infinitive of hear is to hear. So, it should be either hear with the to which turns it into an infinitive as required or hearing without the to:

It is nice to hear from you.

It is nice hearing from you.

Long story short, to in this case is not a preposition like it is in I am opposed to drinking. It's something called an infinitive marker whose sole purpose is to indicate that the verb is being used in its infinitive form. to plays the same role in forming infinitives as ing in forming gerunds. The upshot of this all is that phrases like to hearing where to is an infinitive maker are just grammatically impossible.

Not only did the adjectives derive from verbs and followed by “to” must be followed by a gerund, but there are adjectives (derived from other words) and they still apply to this matter that gerund come after them.

And, I think, those adjectives that come with specifically certain verbs to forms “combined adjectives with prepositions” must be memorized.

• I am allergic to eating apple.

• English language learners site is very beneficial to improving your understanding of English.

• My body health system is immune to getting infected by different diseases.

• Look, in all these examples the adjectives aren’t derived from verbs, but they are still followed by (to + gerund)

You can look into this link to see many of them. the source