I bet they'd move faster if they could. I'm guessing for takeoff there's a good amount of analysis to do beforehand. Terrain, weather, system checks, whatever.
I don't know the actual list, but it's not hard to guess why they couldn't do everything, including ejecting it from the main rover and getting the main rover out of the way, in the ~12-14 hours a day that they have to do stuff.
For example: if you know that lift-off works, then that's a huge datapoint.