See, over a period of time, what happened is that these got bigger and bigger. Nobody ever figured out how to get them to do the stairs because the base is too broad, so you just buy one per level and sweep the stairs yourself, down onto the lower landing for the downstairs one to get. They sold more that way too.
They picked up new duties over time too. A bit of pest control (a zapper) and then plumbing (a big flexible suction cup thing on a rod for plunging toilets.)

It was late, late in the day that we realized the problem, and by then they were already deadly.

Humanity beware! The Roomba is the ancestor of the Daleks.