1. There’s no way in hell Bill Clinton selected Al Gore as his running mate in 1992. It was an open secret in Democratic Party circles that those two despised each other.
2. Reagan had GHWB forced on him by the Republican Party in 1980.
3. Obama didn’t select Biden as his running mate in the 2008. McCain didn’t pick Sarah Palin, either.
In most of these presidential elections, the candidate is a former governor who will doesn’t have insider connections in Washington. The national parties give them running mates who are party hacks as a way of keeping a leash on them.
I’m sure you’re correct re those cases.
But, as I said, it’s not codified. It’s just “past practice” and can be changed.