Fear of losing a bag. Don't want to wait at carousel for bag. Everyone has a reason. A lot of people have reasons. Those are the ones taking up the limited spaces within the airport also with their bags rolling twice as wide on walkways. Jumbled up at seating areas and eateries, restrooms.
Not so. Not all of us. My bag is small. My former carryon (which I still have and may still use on occasion) is a "small underseater." My newest carryon is an "international rolling carryon," sized to meet the carryon limits of most airlines outside of North America. For airlines that are stricter, I have paid the extra charge that permits me a seat near the front and a larger carryon limit.
This is my only suitcase for my trip, and I like it that way, not just for flying purposes. It's also easier to for me, a petite senior, to manage hopping on and off buses, trains, and ferries, walking to my accommodation, etc. It's easier to stow if I have to store my luggage for a while when I have an early arrival or late departure. There are many reasons why I prefer to travel with a bag that size, and if it meets the requirements, what's the issue? Why should I have to check it, if I can be efficient enough to travel for weeks with a small bag?
I do agree that airlines should enforce their limits, though.