It’s like when you lend money to someone and they avoid you to never pay you back. Depending on the amount that can be money well spent.
The problem is the transaction is unbalanced. They got help when they needed and refused to help when called upon. Someone took more than they gave and there's no real way to address the issue so the only sensible thing to do is to avoid putting oneself in such a position in the first place.
It's also hard to measure one's contribution in a relationship (hey, maybe they were a really good listener; or maybe they can't find time to help you move, but when your kids get sick, they'll move the world to get the best possible care for them?), so let's not go and introduce quick-and-simple rules for what definitely are relations on a case-by-case basis.
The problem that you solve is information. You don't know who's trustworthy and who isn't. If I can give a small amount of help to someone in exchange for knowing if they are trustworthy or not, that's probably worth it.
The people that reciprocate you can keep in your life and they become valuable friends. I am more than happy with the tradeoff of sometimes giving people some help that they don't reciprocate in exchange for finding the ones that will.
That's a life skill I had to learn. I was raised believing that if I was a good altruistic person all that goodness would come back to me. Well, way too many times it didn't and it was extremely frustrating until I figured out how to deal with it.
Sometimes it's okay to be selfish. It stops people from taking advantage.