Why are do soo many actors fail, to become great?

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Because of maths. Stay on this site just a month and see how many people share the same dream. See how many go further and train and get good agents and really give it a good go for a few years. And every year they are added to and more want it than give up on it. There just isn't enough work to go around. And those who do work soon realise they simply aren't getting enough work to make a living, and the vast majority give up, statistically within their first five years, but I have known some who live on poverty wages for a decade and lose out on so much in life - socially, having a family, a home they can call their own, and, of course, self worth, until they simply buckle to the inevitable.

Nothing to do with looks - look at all those rather "odd" looking actors there are who constantly work. Then look at all the "attractive" actors who may have a few years of success then fade away as they are replaced by better actors, newer actors, more attractive actors.

The vast majority become these "losers" you despise, but if you don't try you'll never know where you'll end up. There's no guarantees, the odds are against you, and there are no safety nets. But it's exactly the same for everyone else.