Sometimes there is no space left to put your stowed items, once you get on (if you’re not in Group One, say). You then have to find an overhead space where ever you can...sometimes in rows in front or back of where you’re sitting.
Would it have been so difficult to ask around, if that was a passenger’s bag, before handing it off to ground crew as abandoned/forgotten?
No doubt, but when you put stuff under your seat you're taking space intended for the passenger behind you.