I think there are a couple issues at play:
-
Different planes have different overhead bin designs, but I don’t think there are many (if any) planes that actually have enough overhead space for every single passenger to stow a rollaboard. And even if they did, enough people bring on oversized items, or try to stow two items overhead, or put up their bags at the first available open bin (even if they’re sitting all the way in the back of the plane), that on flights with checked baggage fees, there’s usually a shortage of overhead space, especially in the front half of the plane.
-
Flight and gate crews are under a lot of pressure to make the plane leave the gate on time, and dealing with stowing luggage in the overhead bins is one of the easiest ways for them to miss that mark. Pressuring passengers into gate checking their bags (even when it’s not strictly necessary) is a lot more efficient than trying to rearrange luggage on a crowded plane to make everything fit in the overhead bins, or allowing passengers to go to the back of the plane to stow their bags overhead and then salmon their way back to the front to take their seat. Note that Southwest’s emphasis on turnaround time is one of the reasons that they both (1) eschew assigned seating and (2) don’t charge for checked bags.
Note that in this video they are not arguing over whether the bag is too big to carryon. They are arguing over whether it qualifies as a “personal item.” The passengers here have almost certainly bought an ultra-cheap Basic Economy ticket from United, which doesn’t allow you to bring a rollaboard-type carryon, only a “personal item” that will fit under the seat in front of you (which you aren’t supposed to put in the overhead bin at all). United’s theory in offering Basic Economy fares is that it gives passengers more choice (because you can choose to pay a few dollars more and bring a carryon, but you have the cheaper fare available if you don’t need one), but the second they announced it a few years ago it was pretty obvious that it’s mainly a recipe for sparking fights between airline staff and pissed off passengers like these.
