It's because most state schools are at least semi-funded by state taxes, so if you or your family have been paying state taxes, technically you've been paying for that state's university system already, so you're getting a discount that reflects what you've already put into it. So when you go to a school out-of-state, they go "oh hey, you haven't been paying into us like all these in-state folks. Here's full-price to reflect that".

No one said it was fun though. And I don't necessarily approve of the notion, that's just the reason why.

But... alrighty then. I suppose that makes sense on some level. I still think it's annoying. And I now need to make another trip there...