They're also assumed to be better off financially, and thus less likely to steal from you.
People usually five figures in debt from their schooling are better off financially? Sounds a bit backwards. Perhaps more desperate and tolerant of abuse to keep a job.
There's also certainly some of that, but bear in mind that you can still attend a smaller state uni for under $10k a year, not including room and board, books, food, or other ancillary costs. So even someone without good scholarships can at least potentially work their way through school with some effort, without going into debt. It's hard work, but it is entirely possible, and I know people who have done it. Also, bear in mind that most places run a background check, and they can likely see if you are significantly in debt. So that can be factored in directly, without having to use education as a proxy for the data.
Not everybody gets scholarships, and not everybody can afford to pay for college with their trust fund money. And while student loans are sometimes available, depending on your parents's income, your options may be limited, especially if you fail to keep your grades up.
Thus, being able to afford college is at least to some extent a way of selecting for some combination of intellect (scholarship potential), determination (working your way through college), and familial wealth.
Higher intellect likely means better ability to get the work done. Higher determination likely means being more willing to push to get the work done. Wealth likely means lower risk of white-collar crime. And getting through college likely means that you are at least organized enough to not completely fall apart, have moderately competent time management, etc., all of which are beneficial.
Given a choice, a hiring manager would prefer someone with some of those characteristics over someone who has none. It's a fairly weak signal, but at least arguably, a weak hiring signal is better than no signal at all. Or at least that's what businesses usually claim when asked.