There seem to be two ways to make big money in Hollywood, either be able to draw a large audience or be a good self-promoter.
Nicole Kidman, for example, I don’t know that she ever made any movie that people wanted to see because she was in it. And yet, somehow, she is a name that people know and has had steady work in the industry for 20+ years. And, I suspect, she earned herself some decent salaries by simple process of being a ruthless negotiator at every step in the process. I assume that she has an agent, but most likely the agent just fields phonecalls and idiots. The real heart of the engine is Kidman herself.
Amy Adams, people want to go watch her films. They trust that she’ll have picked a decent script that, even if serious or dark, still does it in a classy way.
Michelle Williams, I’ve never heard of. It doesn’t sound like she’s put up much of a fight to see to it that she’s getting well-paid.
Wahlberg didn’t get a payout because his agents care about him more than they cared about others, it’s probably just because Wahlberg is a dick, didn’t care about Kevin Spacey one way or the other, and knew that he had the studio over the barrel. He knew that he could get the extra money, so he got the extra money. That’s just how his mind works.
An agent isn’t going to be able to make a decision like that. What if they decide to play tough and ruin Wahlberg’s reputation, get him written out of the script, or otherwise screw him over by trying to play hardball with the studios? That won’t fly for a business. It has to be Wahlberg himself making the decisions.
So, really, this is a matter that Wahlberg cared more about his finances than he cared about what Kevin Spacey did. That’s not admirable, but it tells us more about Mark Wahlberg than it does about the studios or the agency.