Because that's not always an available option in the housing market. There are some cities (for example, present day San Francisco) where city wide apartment vacancy is so low that tenants basically have to take what they can get. Tenants are at a serious disadvantage to the whim landlords in this kind of market. Thus laws exist that restrict what landlords are allowed to provide, which serves to protect the tenants.

You can make an argument about not restricting the free market, but shelter is such a basic human need that I think it merits a healthy amount of regulation.