They are not properly structured.
Mortgage companies control rent to own rules. And the biggest one talks about how much of the rent can be applied to the purchase.
The answer is: any amount over and above the market rent for unit they are in.
For example: if rent in the area is $1000 per month for the same type of unit and you pay $1200 to the owner ever month, $200 can be counted towards the purchase.
And that is why they fail.
There are a small amount of people that have their situations change when it comes to size or location.