Being one of these people, I think I can explain why. First of all, as Milly mentioned, you can arrive on a tourist visa and switch to a work visa without leaving the country, and for obvious reasons, it's easier to take a temporary job teaching English and job search in the city rather than online from abroad. Also the company would have to do a lot more legwork to bring someone from abroad, whereas the job seeker can do it being here. So logistically, depending on one's timeline, coming here on a tourist visa can make sense. I think this is very different than coming here expecting a job to be waiting, not speaking Spanish or having marketable skills.
And a message for other young expats and for EMPLOYERS, at the risk of sounding ridiculous: it is not that difficult to get a work visa. It is tedious and takes a lot of paperwork and waiting (welcome to South America), but this is nothing compared to the kind of strict qualifications and quotas surrounding North American work visas. The employer only has to sign a pre-contract that is not even a binding employment contract. So I think it's equally ridiculous that so many employers create a chicken and egg problem by refusing to even consider people without a DNI in hand.