Why do celebrities get high wages?
If you ask yourself how much a potential employer would have to pay you to convince you to work for him, the answer is probably something like "at least as much as I could earn by doing the same job for another employer". So, provided there are several employers competing to hire workers, you can think of employers as bidding against each other for the best employees. But how much will firms bid? Each employer would be willing to pay up to the difference between its profit if it hired this particular worker and its profit if it took the next best alternative. 

This is why the supply of alternative employees matters. If there are many people capable of doing the same job then a firm's profit if it hires you will be almost the same as the profit if it hire someone else. So why should it 'bid' a lot to hire you? An unskilled factory worker who asks his employer for a significant pay rise will likely be refused because virtually any worker doing the same job would have equivalent productivity.

Celebrities are a different story. Just as factory workers are hired to produce goods for the factory owner, actors are hired to produce movie ticket sales for the studio. But viewed in this light, Tom Hanks or Angelina Jolie are not close substitutes for as yet unknown actors at all. Indeed well-known actors are considerably better at producing ticket sales than unknown talent. Studios are willing to bid a lot to attract top actors because having a star in your cast is worth millions of dollars in extra ticket sales.