You're looking at students at a top tier university in a field that pays extremely well. The numbers are going to be at the high end for what a grad student can make. A quick search for PhD salaries suggests that $20-35k/year is more common.

The median wage number you cited is also for the total population. According to this graph the median wage for college graduates is around $7k/mo. I'm fortunate to make very good money but I'd still notice a $2k/mo pay cut.