"Why would someone do that?"
Simply because the jobs I have had never had vacation days, sick days or any other kind of paid day off other than weekends. Even on the weekends I work mainly because I get bored doing house chores and fixing stuff I don't care about. I like bikes so I spend Saturdays fixing them at the LBS. Before that I ran a side biz promoting sales events, and before that the side biz was contracted to Walmart's Sam's Club. Time is coming when I can't do all these things, and will wish I could.
Wish I had a job that paid me to do nothing, but I don't. Such is life. Any time off is money out of my pocket.