For starters, there's a lot of people who simply weren't born when their countries joined, and they simply don't know what it was like when you had to pay custo

For starters, there's a lot of people who simply weren't born when their countries joined, and they simply don't know what it was like when you had to pay customs fees to bring stuff across that which is now internal EU borders, when you had to show a passport to cross European borders and so on. As someone else posted, this is one of those "What have the Romans ever done for us" things.