1) Not everyone can do that, physically or mentally
2) Costs have increased so much, wages have not kept up to allow that to be possible for most people: http://www.vox.com/2015/8/28/9220705/college-working-map
I paid my tuition almost entirely through co-ops (6th month internships) and then worked while in school to pay my day to day bills. I was also very lucky to have a safety net from my parents if I needed it, although I didn't.