They don't. Full siblings (non-identical) can share between 30% to 70% of their DNA, while a parent/child will always share between 49% to 51%.
The trouble is the number systems only read one chromosome position match, although you actually have two positions (one maternal and one paternal) along each chromosome, for a total around 7200 cM.
So your mother will match you across all chromosomes at about 3600 cM, but is only matching your maternal side of that Chromosome. This is called an HIR (half identical region).
If you compare you to yourself, it will also read as 3600 cM. But it is matching both the maternal and paternal sides, known as FIR (fully identical region). So the actual cM is 7200, but the companies don't distinguish between HIRs and FIRs, so report it as 3600 cM.
So if you compare yourself to a sibling, and get 2800 cM, that is the minimum HIR regions. There could be an additional 1500 cM FIR regions, where the sibling matches both your maternal and paternal in the same position that are not being counted. So you actually match at about 4300 cM.
Both of my siblings match me above 60%, but the company reported cMs are closer to 40%. I think 23andMe is the only company using the 7200 FIR scale for comparisons. All others limit themselves to the 3600 half scale.