That said there is a fair about of wiggle room so I doubt it’s a significant issue in practice.
I even have a cool name word to describe it: "lid"!
But even beyond that feeling like you’re falling out of your seat is unpleasant.
You do sometimes see cup holders on private airplanes, but in commercial aviation the efficient solution is to avoid needing them.
Similarly, adding decent cupholders to airplanes is not going to reduce passenger space.
As to why no cup holders, people aren’t going to use them most of the time and they don’t even help that much in turbulence.