Industry standards. These are jobs that, when you're starting, has probably the least regular hours you could ever expect, with a constant demand of professionalism and flexibility. A lot of entertainers get into hard drugs just to keep running at the pace demanded of them. You've also got an industry of volatile people, often the people behind the scenes are more unstable than the stars themselves. Unless you make it really big really quickly, you can't afford to be unstable as an aspiring star. This is not the case for those in power behind the scenes, and they set the standards of working conditions.
Of course, it's one of the few industries where there is the money to bankroll this lifestyle, even if half these stars are in inescapable debt to their masters.
Another thing is that it is an accepted and expected narrative, the downward spiral. Money gets made off of it in the media. Some stars are worth more dying or dead, so it gets encouraged.
Finally, there is a selection bias. We remember the crazies. We remember things that stand out, so the number of these seems disproportionate. When's the last time you heard about Mike Patton's drug problems? He's probably more creative and talented than most of the celebrities one thinks of regarding downward spirals.
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