> state schools start around 15k/year and provide bachelors degrees.

Problem 1: School is expensive

15k*4 = 60k (assuming you do 4 years and not the average time of 6 years). So we're pretty close to $100k of debt. That is just in-state tuition! But that's not the total cost of school. Berkeley[0] estimates about $30k/yr for a student. So that's 120k-180k of debt for a bachelors. Now double that if you're out of state.

Problem 2: School is getting more expensive

But there's another problem that's happening. Cost of tuition is skyrocketing. Let's look at Berkeley from 2003-2012[1]. In 2003 the cost was $4336 (adjusting for inflation today that's ~$6050). In 2012 the cost was $12,834 ($14,340 2019 dollars). Today it is $14,184. So in 20 years the price has about increased ~2.5x. This means that those cases of $100k+ debt are increasing not just in total number (because population increase) but as a percent of graduating students (we'll say Gaussian curve of debt distribution).

Problem 3: In-State isn't always the best option

Let's say we live in Washington (state) and want to go to school for computer science. The field is fairly competitive so the higher ranked school we go to yields likely a better education (one up to date with modern techniques, funding, and equipment) and more network connections. So we'll say that we want to get into a top 100 school. That actually only leaves us 2 schools: UW(#6) and WSU(#75). We graduated pretty well with a 3.9 and a 1400 on our SAT. That might not get us into UW ([3] says 33% chance we don't get in) because it is a (inter)nationally competitive school and we don't have really any extra curricular activities. So what do we do? Our scores are high enough to get into a top 20, worst case top 50. Do we go to WSU or go to a school with more resources and a higher chance of earning more in the future? And if you think this is a fictitious scenario, play around with the other school acceptances (website uses same naming pattern as usnews: e.g. University-Of-California--Berkeley), UW is one of the least competitive schools in the top 20. There's plenty of states with even larger gaps. There's plenty of states where we're more competitive than the top instate school. There's plenty of other reasons a student might want to go out of state, and plenty of reasons that might be the best option for them.

Problem 4: Age

We're also talking about 18 year olds, which aren't known for making the best decisions. I don't know about you, but when I was 18 I was an idiot. I'm still pretty dumb but I've learned a lot since then. And if you don't think you were an idiot at 18, I'm calling bs or that you're under 23 and just don't know it yet.

[0] https://admissions.berkeley.edu/cost

[1] https://4.bp.blogspot.com/-wswpT9L7QXs/UMjqxuIbttI/AAAAAAAAA...

[2] https://www.usnews.com/best-graduate-schools/top-science-sch...

[3] https://www.prepscholar.com/sat/s/colleges/University-of-Was...