Answer:
because they live in stables hahaha Well, chemically, humans and horses have a very similar composition. After all, we are made of the same basic biological molecules—fats, carbohydrates, proteins, etc. So chemically, our stability is very similar.
I realize that you probably meant tendency to fall over, and it is just my chemistry-oriented brain that supplied the above answer.
Horses have four legs to support themselves. Humans have two. This gives humans the advantage of hands and all the things you can use them four (imagine trying to climb or pick something up with hooves). However, having hands sacrifices the stability of four legs.
As for why four legs are more stable than two, consider:
Something falls over when its center of mass gets to one side of the base of support (Well, it is more complicated than that but anyways).
Step-by-step explanation: