As a business, WWE is not really any different than anything else. You have corporate management that weigh every decision based on what they think will be the most financially stable, you have employees who aren't corporate that don't know what the inner most goings on are as far as business goes and most employees get nervous whenever any degree of financial trouble is looming because it means people getting laid off
Also as with any other business, you're going to have some employees who're disgruntled and some employees who aren't. The ones who aren't disgruntled are the ones who make out the best, from a financial standpoint, and who're viewed as the top money making talent for the company. Those who are disgruntled are usually those who don't make as much money, though they still make a good living, feel that they're underappreciated, feel that they deserve more than what they're getting, etc. As a rule, you're gonna have more disgruntled employees than satisfied ones.
Navi mentioned the Screen Actors Guild and any of those guys could join if they wanted to. However, it's not required for any actor to join SAG if they personally don't want to. The criteria can be found on SAG's Wikipedia page. Essentially, if you're an actor, have had verifiable jobs as an actor and can afford the initiation fee and annual dues, you're eligible to join.
If The Rock ever became president of SAG, I wouldn't see him doing anything to step on WWE's toes. The Rock is one of those guys that's obviously done extremely well and WWE has been the cornerstone of his success. As a result, he'd be more likely to sell his mother to a sideshow than attempt to do or say anything to hurt WWE. At the same time, just like most other organizations associated with mainstream media entertainment, I wouldn't be surprised that wrestlers who occasionally act in low budget, straight to DVD movies aren't high priorities.
Also, the guys Navi mentioned are also the guys who're less likely to raise a stink over anything because they've done well. Of those guys, only Santino isn't or hasn't been a top guy but if the various reports on him are true, he's quite content & satisfied with his role in WWE and the money that it's brought him. Santino's always done whatever's been asked of him without complaints, without gripes and, as a result, he'll probably always have a job of some sort in some area in WWE for as long as he wants it.
At the end of the day, most pro wrestlers are 100% selfish in the sense that their primary concern is for themselves. In order to organize into some sort of union or genuine walk out, they'd have to be willing to genuinely behave as a brotherhood that's looking out for and caring for each other and I don't see them doing that. Like a lot of businesses, wrestling is pretty cutthroat and there are always a good deal of wrestlers who're looking to improve their lot in life by stepping on the backs of others. At one point, pretty much every major star in WWE or anyone that's ever really been a somebody in pro wrestling has done that. Thesz, Sammartino, Race, Hogan, Flair, Sting, Savage, Warrior, Hart, Austin, Taker, Lesnar, Cena, Orton, Triple H, HBK AND CM Punk have all played politics to get ahead or to at least attempt to get ahead. Triple H said it best several years back: "In this business, you can make friends...or you can make money."