If someone knows a formula to success. Why would they want to give it away to mentees?

Note: I’m talking about FREE mentorship. Those mentors who pay back to the next generation.

I’m genuinely want to understand, if a successful person knows the exact formula how to build wealth and get rich. Why would they want to share it to other people or even have mentees? Why mentors exist if they don’t tell their mentees exactly how to be successful? I’m genuinely want to understand if someone who is very successful in a particular field why would they want to share the secrets? Would it just create competition instead? Normally I assume they would keep the secret/formula to themselves only. Or use the time to build more wealth? Please help me understand!

Next Question is: Where do I find such genuine mentors? Is it by joining programs?