Actors who are just starting out in plays, movies or TV don't make much money. Even worse, it's an income stream that's sporadic. Therefore, they have to support themselves by working jobs that will give them the flexibility to leave at a moment's notice for auditions.
A lot of film and theatrical agents won't take on new clients unless they already have an impressive resume of acting credits. Unfortunately, prestigious roles rarely go to actors who weren't referred by an agent. The same with actors' unions such as the Screen Actors Guild; you can't get in unless you're making a certain income as an actor but are precluded from trying out for some roles unless you are in SAG.
It's hard to be in a relationship when you're rehearsing Monday to Thursday nights and doing shows on Friday, Saturday and Sunday. If you're in a touring production, you could be apart for months. If you're in a TV show or a movie, your day usually starts at the crack of dawn and ends after the sun goes down. Whatever time is left, you'll just want to sleep.
Just as in the modeling industry, females tend to have a short shelf-life in Hollywood unless they are either adept at character roles (i.e., comedic supporting players) or have the ability to gracefully grow into age-appropriate roles and constantly reinvent themselves. In contrast, the theater respects maturity and always has.
Even actors who win Oscars, Tonys and Emmys still wonder where their next acting gig is going to come from. Even actors who land on a hit TV series live in a constant state of angst that the show could get canceled or their character killed off. To stay on top of your game, you have to keep honing your craft, learning new skills, and keeping your name and face in front of casting directors.
Actors will sometimes do anything to get a coveted role. These extreme measures include such actions as gaining weight, losing weight, shaving their heads, growing beards, financing the entire production, and succumbing to the charms of the director.
The proliferation of college and community theaters and independent production studios throughout the U.S. has made it happily possible for many people to indulge their fantasies about acting on the side without having to sacrifice the security of a paycheck, health benefits and all the creature comforts of home.