I think the difference is that you can make software using just a computer, whereas for a robot you need to order a bunch of physical parts and put them togethe

I think the difference is that you can make software using just a computer, whereas for a robot you need to order a bunch of physical parts and put them together. The cost is much higher and you can't just tweak physical parts like code. If you mess up, you have to buy more stuff. Also to make truly top quality robots, you need to know how to weld and cut metal, or mold plastic. If every kid had a warehouse in his backyard full of robot parts and tools, it might look more like the FOSS movement.