Part of the issue is if you fly on a region US domestic flight, they figure everyone is gate checking most bags anyways.
Then that same schmuck with a huge laptop bag, overstuffed gym, and locker on wheels goes to their mainline flight and expect to bring everything on board.
I've seen two near fights on planes in the last couple weeks between PAX over luggage issues.
UA is to blame. Checked bag fees have made carry-on abuse worse. Either stick to the requirements of have none at all.