Well, that's not exactly true. Sure, the production style may be somewhat similar (monologue with various pictures flashing by on the screen representing what is being discussed), but the content and the nature of the discussion are quite different among them.
Yahtzee does reviews/analyses of games that are meant primarily for humor and to pretty much tear apart every game he plays, even if he personally likes it. The worthwhile things about games you can take away from Yahtzee are where games (in his opinion) falter in some aspect of their production, and whatever suggestions he has to improve them. Though again, Yahtzee in his ZP videos is primarily a funnyman, and he gets more "serious" in his commentary about gaming in his Extra Punctuation column.
Extra Credits looks at gaming from the perspective of a game creator (more specifically, a game designer), deals with issues in gaming outside the games themselves, and is heavily into the push for games to be recognized as an art form. While MovieBob and Yahtzee may or may not agree with James' perspective on games as art, they rarely, if ever, bring the subject up in their videos. Yahtzee does not take his gaming commentary outside of the games themselves, and MovieBob, well, I'll explain him below. Extra Credits videos are always serious in their subject matter and often provide perspective on games in relation to other social issues going on in the world today, a perspective rarely seen in gaming commentary, and one that is really appreciated by fans of the series. I recommend you watch more to see the difference.
MovieBob doesn't really do videos about games here. He does the Game Overthinker series on his own site and ScrewAttack, but he doesn't really talk about games here. Escape to the Movies is about movies, and The Big Picture is about... whatever he wants to talk about. He's mentioned games there, but not often, most often talking about comic books, TV, or just "nerd" culture in general. He's mostly serious in his videos, but not as serious as the Extra Credits team, throwing in some good humor from time to time. He's also probably the most controversial of the three producers you mention (though probably doesn't push the envelope as much as Jim), in that many gamers, and even fans of the show, will find some opinion of his they disagree with, much like Yahtzee, but he doesn't play such disagreeable opinions to comedic effect like he does. He is also prone to making big claims and backing them up in a way that seems more confrontational than the Extra Credits crew. Personally, MovieBob is my favorite commentator on the site, as I do often agree with him and I like his style in production, but many would disagree with me too.
So there is quite a bit of variety among the three in terms of content, and that's really what most fans focus on in their videos. Not many people probably watch the videos for Yahtzee's latest variation on the Imp, the new bean-person Alison dreams up (though her artwork is nice, and it was really a great story hearing about how we came together to help her in her time of need), or MovieBob's latest face in The Big Picture. We want to see and hear what they have to say, the unique perspective each brings to the table. That's where the differences lie, and that's really what's important, isn't it?
