I think its more likely that the people do know they just don't care and it helps them to put their backpack overhead so they do it anyways. There is minimal/no enforcement.
If I could walk from the plane to the luggage area and my luggage was already there 90% of the time, I probably would check more things.
However, the US airports simply don't employ enough people to move the luggage around fast enough.
The is 100% correctable by employing more people. But some CEO needs another yacht, so they don't. So, I simply don't check luggage.