I can only speak for myself, but if a YC or 500 founder I don't know emails me, I'll make helping them in any way I can my top priority.

I am not altruistic by nature, so this is done purely for self-serving reasons:

1. Everyone I've met through YC or 500 is absolutely brilliant at something in their field. I usually end up gaining much more wisdom than I share.

2. Top tier funds like YC or 500 are very selective, so I know anyone I meet with has a nontrivial chance of being one of the world's top entrepreneurs in a few years. It pays to get to know them before they are famous.

In short, meeting with someone endorsed by YC or 500 is guaranteed to be worthwhile. That removes a lot of risk in choosing to invest time to help them.