Not everyone can. Here in the US, doing so without family support / fallback if you fail out makes it extremely risky. If you're supporting yourself completely, having to work gets in the way of going to school (immersion while working full-time, especially at workplaces that demand all your time is basically impossible.) Others succeed in a work setting, but struggle in school. Still others don't want to deal with mounds of debt, and have no college fund/trust fund/aren't good enough to get scholarships. There are lots of reasons.
https://medium.com/@opirmusic/why-software-developers-should...