As a person who has mentored people and is currently a mentor, I can name a number of reasons-

* to contribute to my company/community * to make myself a multiplier (I can do work or I can help 5 junior engineers be a little better at their work) * to learn myself (a great way to learn is by teaching) * to grow my breadth of knowledge (I hear about what other people are doing) * to grow my network * to improve my leadership/managerial skills * because people have invested time and mentorship in me

It's not really for free. There is a social expectation that you'll pay it forward by mentoring someone else in the future.

Also, the mentor gets a competent colleague who can meaningfully contribute in return for their time.

Why wouldn't you want to mentor someone for free? Passing down experience/wisdom to the next generation is rewarding in and of itself.

I want to see cool ideas being built, I don't have time to build them all. Helping new founders is a way to do it.

Not all that seems free is free. Mentoring your team members, for example, results in better performance, more income, and larger TC eventually.