* Most parents with means contribute several tens of thousands, at least. If yours don't, they come from debt.
* Financial aid formulas at excellent schools that are just a step or two below the Ivy league have what we call an "awkward middle." There are lots of rich families for whom $60k/yr is not a significant hardship. There are also a fair number of poor families whose children get full rides (+ allowances for personal expenses.) Where you see huge debt is in the middle class.
Expected Family Contributions for middle and especially upper middle class students (let's say $60k-$100k household income in Midwestern suburbia) are often technically possible, but not actually feasible. The number that Financial Aid gives you will be significantly less than $60k. It might be $50k, or $40k, or even $30k.
But that might selling the house, gutting retirement accounts, forgoing college for younger siblings, or even a combination of the above. So the gap between "actual family contribution" and "expected family contribution" means you will need to cover the difference with loans.
* Financial aid roughly correlates to endowment which roughly correlates to prestige. At lower-prestige but still decent private schools (NYU, GW, etc.) it's not remotely difficult to rack up $200k in your 4 years.
* Some people have trouble adjusting to college life, or to the stress of later semesters. An aborted semester due to (potentially involuntary) leave of absence to deal with mental illness, etc. is really fucking expensive, as is an extra year to make up for classes you failed or a change in major. But if you've already paid in $60k, you're not going to just walk away and write it off; you're going to keep paying whatever it takes to finally get a degree that's worth something.