This is because rent to owns are not common and that is because they rarely work out and because owners want to sell and not become landlords of properties that won't sell. Just rent until you can buy - Then buy. You will come out better off if you do.
There is a reason;
Lease/Options are very rare, and,
they are tough to negotiate.
There is no STANDARD Contract for a Lease/Option;
It can be written a million different ways.
It is a real CAVIAT EMPTOR.