Disclaimer: not a physicist. Maybe because a Roomba weighs ~9 lbs but a sofa or table weighs 2x to 10x more. I can answer your actual question by pointing out

Disclaimer: not a physicist. Maybe because a Roomba weighs ~9 lbs but a sofa or table weighs 2x to 10x more. I can answer your actual question by pointing out robot vacuums lack arms or any feature capable of lifting anything. That’s “why” they don’t lift things.