There's something I'm really failing to 'get' concerning overheads. Given that there is one overhead per seat, and that they seem to handle the correct number of correctly sized bags, why do we run out of overhead space?

It seems to me that if they enforce carry on bag size, that they could do so in a way that guarantees sufficient overhead space with not too complicated correct placement of bags. So, why not?

I was on a flight Thursday where Easyjet made a lot of people check their carry-ons. (Not me fortunately).