Unions have no value for 'actors making millions'.
The union is basically irrelevant to them.
The various unions and guilds in film/tv production are there to protect the regular workers who operate in often very irregular and ad-hoc circumstances.
Also - acting is a hugely oversubscribed job, most actors don't do acting full time. There are 10 actors for every role. Even good one's.
Even just 'working actor' - let alone 'millions' - is a very lucky position.
'Famous' actors are not helped by the union, they have agents who manage their deals, much like athletes and their comp. is a function of totally different things.
'Athlete's Unions' are actually advantageous to them, because they can leverage power against owners, but in that case it's 'rich' vs. 'rich' entities fighting over surpluses, and nothing is gained or lost for the rest of us, it's not a helpful basis of comparison for working-class unions.