For one thing, it’s the only line of work where age, sex, and racial discrimination is legal. You may be the best actress in the world, but you’ll never play Hamlet. You can’t have a black actor play a 1940s crime lord. (Of course, if the part calls for an angry, yet righteous man, whether you’re white or black, you’re going to lose the part to Samuel L. Jackson.)
On top of all that, they can discriminate against you just for being ugly.
Even if you get the part, the end product is mostly out of your control. Sure, actors often get credit when they don’t deserve it, but the flip side is, they also get blamed. Did Speed Racer fail because Matthew Fox isn’t a star? No, it flopped because it kinda sucked (although, to be fair, it’s not as bad as you’ve heard).
Sure, there are actors like the cast of Tropic Thunder (and the characters they play! Zing!), but most are just doing the best they can with what they’ve got.
Then again, there’s this guy.