Why are US Regional Airline starting wages so low?
Well, the short version is: The airlines pay what they have to pay in a free market.  They can get away with it because pilots are willing to accept it.  Like any other business they are in it to make money and keeping costs down is just as good as making extra money, so they aren't going to volunteer anything extra.

Now, why do pilots do it?  Because of the promise of better things down the road.  It isn't as good as it used to be, but after being at the airlines for a few years, you can make a livable wage and as you get further into your career at a major airline, you still get paid pretty well for a "part time" job.  As I've heard other pilots say before:  It's better than a "real" job!

Other pilots do it just because they love it and can't imagine doing anything else!