Because there are people out there looking for work. It's not trivial for all programmers to find a job. I am in particular really awful at the entire applying and interviewing process (except funny little algorithms problems, I can kill those). It's a stressful, amorphous blob of everything I hate.

What the parent of your comment describes sounds a lot like a specific kind of contract-to-hire, which if it's done well can be really effective. I get to show off that I can actually develop software well, a company gets to judge me on more than just an hour (or maybe a day) of questioning.

Unfortunately it can be pretty exploitative if done wrong. It's easy for it to become an unending period of low-paid work where you're just hoping that one day it'll turn into a real job.