Until Anthropic and OpenAI IPO, it is no brainer to join and it is almost risk free to get equity and make a huge amount of money before they IPO.
Just wait until the IPO is done and the lockup period is over, and you will see lots of star employees leaving, some making their own VC firms, some retiring or some just making huge expensive purchases.
Nothing wrong with this, but it is a huge reason why they these "well known" tech execs are joining now.