On earth, if the driver of your self driving bus gets too sick to operate the vehicle or gets hurt in an accident or dies, there are 100s if not 1000s of people who can readily replace them.

On Dragon, if something happens to 1 or both of the designated pilots, there are only 5 people who could replace them.

You know how you still need a licence and to be physically capable of driving when in a self driving vehicle because they can still fail and need you to step in? Same thing here. Except on earth if your self driving car loses control and needs you to step in but you’re unresponsive, most are designed to start slowing down and pulling to the side/parking and if they don’t pull over successfully you’ll still most likely live any accident you get into while it’s slowing down. In space, even a minute of you being unresponsive can result in you getting pulled too far into an orbit where you’ll then either crash into the celestial body or get sling-shot off into deep space. The fuel margins are also so tight that you really don’t have any extra to burn correcting coarse even if you’re not pulled into an orbit.

Astronauts have strict requirements for very good reasons. It’s one of the most dangerous jobs you could get into, right next to being in the emergency response crew for nuclear reactors.