Exactly. If the rules say you can have "one carry-on bag + one small personal item", people stretch it to mean TWO carry-on bags, one just slightly smaller than the other. And of course neither of them fits under the seat. Or the second item is something bulky or fragile, like a piece of sporting equipment, an acoustic guitar or an Easter basket. The bins fill up fast when they're filled with guitars and wicker baskets.Lots of people engage in selfish, and self-serving, behavior without regard to anyone else around them.
Flight attendants hate people with slightly oversized bags, because it makes their life harder. Frequent flyers hate people with slightly oversize bags, because they are presumptuous and selfish. And the poor person who can’t get their bag in the overhead bin because somebody else’s bag is taking up two spaces, probably hates them too.
I am sure some airports are worse for this kind of behavior than others. MCO, for example.