Apartments aren't really comparable to houses. They're relatively small units which are part of a larger building. The better comparison would be to condominiums, but good luck even finding a reasonably priced condo in most parts of the US. I'd guess supply is low because there's a housing shortage and it's more profitable to rent out a unit as an apartment than to sell it as a condo.

It seems to me that most people rent because 1) they only need the thing temporarily or 2) there are no reasonable alternatives for sale.