Because they're providing you a service and their time for an exchange of money not signing up to your company for life?
That person didn't say they'd leave, they said they want to learn something, you took it as they'd want to leave which says more about you and your interpretation of it than anything else.
No, I have specifically told every interviewer that I wanted to learn X and Y during interviews (and I had no intention to leave).
To be precise it is more in the context of founders and this text from the message above "see how a successful company operates". Basically if you go there and say "hey I am a founder and I need to learn how X works in a successful company"
Anyway I definitely extrapolated this, I doubt anyone would say it this blatantly. Just --if--.
I think that is more of a personal experience. I've worked in startups for over half a decade now and founded two. I've never been turned down on the basis of my founding experience or my desire to learn certain skills.
The company I'm currently at even knows about the work I'm doing outside the company and supported me as I was working through the latest YC application rounds.
You want good people to join your company, it provides brand recognition if they then go on to found or do amazing things even better. You should never assume when hiring someone they'll stay for any length of time, obviously you want to try and create and environment where they want to stay but if you got a year or more out of the arrangement I consider that successful. That's why you need strong documenting and knowledge transfer policies.
The days of staying at companies long term (>3 years for senior people and >1.5ish years for regular folks) really isn't a thing any more at least in tech and startups.
There are no structures in place that make it advantageous to stay and 10s of thousands of dollars on the table as a worker that make it advantageous to shop around.
Plan accordingly as a manager and things will be a lot easier instead of losing talent because your worried they'll want to do other work at some point which is pretty much a given for most people.
That person didn't say they'd leave, they said they want to learn something, you took it as they'd want to leave which says more about you and your interpretation of it than anything else.