While these salaries may be dramatically different than what the doctors make when they finish residency, you'll see they're decent.
That being said: as a first year resident I once did the math and found that based on the number of hours I was working per month, I was making less than minimum wage.
Could we pay residents better? Absolutely. But, my suspicion is that people would still want to be doctors even if you paid residents half of what they currently made. I don't think medical school admissions would sky-rocket if you paid residents twice as much as they were making right now.