People do it because it's what they've always wanted to fly above all else, and they don't care about minor details like pay. Or they hear about long haul pilots getting six figure salaries for two days work a week, and the fact that those are a small minority doesn't faze them. Or they're doing a job they hate and...same applies. And if you're at a flying school where a lot of people are going commercial it starts to all seem normal anyway.
Ultimately it depends on your priorities I suppose.
Rotary versions of the above:
Q. If you fly a helicopter, you must be rich.
A. No, but I'd be reasonably well off if I didn't fly helicopters.
Q. You fly both fixed wing and rotary? You must be the rich woman I've been looking for to fund my training!
A. No, I've spent all my money on flying and I'm looking for a rich man to fund mine.