>I could never understand why parents would allow able-bodied adult children to live with them

Agreed, just kick your kids out onto the street and let them fend for themselves. I got mine, they can get theirs. Right?

Why don't they just go buy a house or something?

As an able-bodied adult child who lived with his parents for 2 years, it absolutely did me favors in that I was able to save a small down payment for a home. Some people do need tough love, and others don't. It's a case by case basis.

Because a single person doesn't need a separate house. In many countries, the default is for children to stay home until they're married. Now, if that person is like 30 and doesn't appear to be going anywhere, maybe that's a problem.

Hopefully, it's to help enable their success and wellbeing in such a way that it gives them steps towards becoming self-sufficient. Your tone is slightly crass, but you're not wrong that enabling bad behavior in your children is in opposition to your goals as a parent. If you can reason that your nourishment of resources gives them opportunity without becoming a direct dependence and the cost to yourself isn't too high, I don't see it being a problem. Essentially, it varies on how you see your role as a parent and your relationship with your child.