Because there are not many desirable traits. If people were really doing what they enjoyed, it would be fine, but the problem is that they often don't. I am thinking more about the video game industry, but I think there is a similar problem in animation and all other "fun" industry.
People who get in these industries often want to get paid for what they already do for fun, being doodling or coding small games. Or maybe they see the big projects like a popular anime or a game they loved and want to be part of it. But in many cases, what they only get is a repetitive, not very creative, and high pressure job, with all the fun sucked out of it. Which would be fine if the pay was worth it, but it isn't, and the prospects are not great either, because the really interesting positions are few compared to the grunt work.
Problem is that newcomers don't know that, or maybe hope they are special, and companies really play on that, schools too. What happens is that a few years later you get a bunch of guys that are completely drawn out, hopefully they find a better job elsewhere, to be replaced by a new batch of kids with a soon to be broken dream. Some are luckier and get the job they really wanted (it probably won't be you), and others are really unlucky because their skillset doesn't translate well to other industries and end up with burn out, depression and poverty.