You have enormous power over people who have next to NO power. You can make a nobody…somebody. Sort of. If they have no talent and you hold them out as being the next Meryl Streep (i.e., has lots of talent, doesn't seem to be a wacko, is a total professional who directors and producers love to work with…), and they turn out to be the next Jonathan Morland (Who? Exactly.), your rep gets hit. The next time you pound the table for a new client who really DOES have talent, your poundings may fall on deaf ears. So there's a balance here and "rules" for how you have to use and abuse your power over nobodies.
But over already-successful talent who is proven, you more or less have no power. Or at least very little. Like…what's an agent going to do to push around Brad Pitt or Steven Spielberg or the guy who did Family Guy? Not much. Those successes can get more or less any agent they want and they may not even need one—what studio exec won't pick up the phone for a call from Brad?
And remember, all day long you are selling to people who generally don't need you—there are always other agents, other talent or the ability to just say, "Nah, we’re not making the movie/tv show/series/show/whatever…." The power to say "no" is huge in Hollywood.
There are exceptions, of course, and times in history when agents had tons of power. For a while when movies were a growth industry (i.e., home video and DVDs were actually selling), there was one big fat agency called CAA which had two-thirds of all the wanted (not in an FBI way) talent. So they could raise prices with impunity. And they did. And their clients loved them.
But then the industry began its steady decline and well, bad things happened to bad people. Okay, maybe some of them are good. Power and karma are odd bedfellows.