One small though maybe not solid reason is that the mail is so inconsequential that kids don’t care enough to make the change.
Here’s an example of mail that I still get on the regular for ds1: When he was a junior in HS, he did an internship for a state agency. They enrolled him in a state retirement plan for his three months of service. He still gets quarterly statements from that account, which is worth less than $10. This is not a pressing issue for him or me so it’s continued to ride. And my guess is that’s the reason we got some random piece of mail last month from that other long-ago employer – this was his permanent address at the time he was hired there, and he hasn’t taken the time to change it. Laziness more than anything, I suppose. Again, not a solid reason but one that makes sense to me.