Some of the comments here make me think of crabs in a pot pulling down the ones who try to climb out. Nobody's making you participate in this venture. If you don't like it, then you're free to go do whatever it is you do like. You might think Musk could better direct his efforts and resources elsewhere, but most other billionaires don't do anything all that interesting, they just invest their money in mundane stuff,…
Simple. They are jealous they are on the sidelines.
There can be criticisms of a plan to send a few people to die in a metal box millions of miles away at incredible cost that do not require jealousy as a motivator.