Because they want to eventually be making money and the only way to get there is to work a job where you get paid like crap for way too long. It's just a matter of degrees. If they pay even just a little more reasonable it would make a huge difference. The very lowest salary for a brand new FO should still be something that you could live off. We love to fly and we'll do anything to chase the dream, and sometimes we don't realize what we're getting ourselves into. That's what happened to me, two years into a regional that said I'd have a one year upgrade and still an FO and technically qualifying for foodstamps, I said "f- this, I'm out". It was a hard decision but the best one ever for me.
Only problem I see is that many people seem to think that the airlines is the only way to upgrade in the aviation world, and the only career path. There are tons of interesting flying jobs/careers out there that do pay pretty well. I agree that regional pilots should make a reasonable wage, but for now they're stuck with the crap sandwich they have. My point is, there are options. People don't have to live a poverty lifestyle pursuing what they believe to be their only option to success in aviation.
Now if they've done their homework and the airline route is what they want to do, then that opens the door to saying "don't complain too much, you knew the rules of the game before you decided to play. Just work on improving it."