I think the implicit take is that if your company hits AGI your equity package will do something like 10x-100x even if the company is already big. The only other way to do that is join a startup early enough to ride its growth wave.
Another way to say it is that people think it’s much more likely for each decent LLM startup grow really strongly first several years then plateau vs. then for their current established player to hit hyper growth because of AGI.
A catch here is that individual workers may have priorities which are altered due to the strong natural preference for assuring financial independence. Even if you were a hot AI researcher who felt (and this is just a hypothetical) that your company was the clear industry leader and had, say, a 75% chance of soon achieving something AGI-adjacent and enabling massive productivity gains, you might still (and quite reasonably) prefer to leave if that was what it took to make absolutely sure of getting of your private-income screw-you money (and/or private-investor seed capital). Again this is just a hypothetical: I have no special insight, and FWIW my gut instinct is that the job-hoppers are in fact mostly quite cynical about the near-term prospects for "AGI".