Honestly I kinda wonder why they don't have co-pilots on buses and trains. Imagine what a runaway craft of whatever type you want to pick would do if no one realized the pilot was dead or incapacitated when it plowed into ... well use your imagination. Another craft, dock, city, gas refinery, for buses, a crowded intersection going 60... plain sedans are 2-ton death machines in the right circumstances. Ever watch those recordings of some disgruntled worker making mayhem in a city with a bus or large semi-trailer or truck?
However, I guess the incident rate for pilots dying or being incapacitated is very very low. So, yeah... it's just the potential for devastation if they dozed off kicked the bucket at the wrong time is huge. Just very unlikely.