It's not to stop you from abusing their systems but to stop scam victims from being screwed
One easy example is that you can get around the "no screenshots" lock many bank apps use with root, allowing you to potentially expose security vital information to people.
Should those of us who know what we're doing be allowed? Maybe.
But it's there to protect the old people who will run the .exe that's designed to root their phone and then let them hand over data that would otherwise be locked down so that doesn't happen just because someone called them and said they're from the bank.