Source: College career fairs. BigCos come and recruit people in hordes. Around 60-70% of people graduating from UT, Austin went to a BigCo (Google, MS, Amazon etc)
Microsoft and Google and such usually had their own separate events, and I'll grant they were more serious about hiring people, though they mostly seemed to hire people who'd already been their interns.
Thus they are just the filler needed by BigCo to compensate from the continuous braindrain. Once they have the skills and know the ropes, they'll either get promoted or leave for an employer of choice.
Too many engineers on this site live (work) in a bubble. The rest of the labor force doesn't have it quite as good.
Big organizations have the potential to be great places to work. How can you criticize the ethics of hiring at large organizations when we see daily accounts here about how terribly discriminatory startups can be with their "culture fit" requirements? I work for a large federally-funded organization. We are quite diverse, have good job security, we all get our own real offices, and we get to work on important problems that actually stand to make an impact in national policy. Sure, meals aren't free, but we also don't have Chet the Brogrammer Boss telling us to stay late on Friday to crush that javascript.
Since almost all large companies are doing this, it means you'd have to ignore half of job offers, and the vast majority of people can't afford to be that picky, especially in the current job market.