WWE is "that" company. Beyond the fame, there's job security and benefits that most wrestlers strive to go for. As much as the indie scene has been thriving lately, wrestlers who chose that route still have to grind out the ass to get funds...but at least they get to see their families afterwards. Some wrestlers don't have that drive to get over in the indies and would rather the E take care of it. On the other end, you have wrestlers in the indy scene that have a booming success equivalent to the WWE. Take the Young Bucks for example, dudes are pretty much rock stars making a killing in merchandise alone. 
Its like this, WWE is like the ultimate dream for wrestlers to be apart of. How many wrestlers have said "gee, I want to get over in Chikara" or "CZW is the place to be a superstar in". Not to knock those promotions, but you know.....