A typical economy seat pitch is 31.5". There are usually three seats on either side of a single-aisle aircraft. That's 10.5" per seat, essentially useless.

The only reason overhead racks work at all is that some people don't have roll-aboards, some only have stuff that squishes into spaces between other bags, some people have long-but-skinny things that fit in front of other folks' long-and-skinny things, and so on. With this rule nobody would be able to use the racks for anything besides an old jacket you don't mind rolling up and a bottle of duty-free liquor.