Because the hard part is figuring out where it actually is relative to these borders when it runs around later. If the border is actively detectable (beacons, buried wire, ...) that becomes easier.

It would still need one or more fixed points of reference to keep track of its position. It wouldn't be able to reliably tell where it currently is in relation to where it's been.

Problem is probably not in boundary setting, but in bot positioning. Anything fancier than beacons would raise production or support costs too much for mass consumer product