I think the main difference is that a lot of people simply don't have the discipline to really save money by themselves. If they're renting, the majority won't

I think the main difference is that a lot of people simply don't have the discipline to really save money by themselves. If they're renting, the majority won't save the difference, they'll just spend whatever's in their bank account. A mortgage is forced savings, and that makes all the difference in the world, for a lot of people. It's not as clear cut as "those who can't, can't". Plenty who can, won't, unless they're forced to by a mortgage or mandatory pension contributions.