[PeterisP]

Because it requires either a team or an experienced multidisciplionary person - you need to do mechanic design and part fabrication and electronics and software.

Focusing on just one of these areas would be suitable for a hobbyist, but that's not really a practical option until/if you get a solid ecosystem of cheap and interchangeable components, which really isn't there; sensors and mechanical parts and tools cost a lot, and complete platforms even more, so if you want to work with anything interesting you generally have to do it as part of a lab or team, not as a lone hobbyist.

Also, there is a very long path to any gratification - it takes a lot of work on all the segments until anything starts working and you get some positive feedback; e.g. on the autonomous snowplow example, it would be a loooong road until the 'hello world' autonomous snowplow that's e.g. able to drive forward while the camera is showing all white, as that requires all your engine work and driving parts and electronic driving controls and camera integration to be in place, and if any of these things fails because you're not skilled enough, it just won't work at all. Contrast that with many hobbies where you can get some visible (even if sloppy) stuff done relatively early when starting out; robotics has a very high barrier of entry compared to other hobbies.