At no point in the movie does anyone take off from the Martian surface and escape Mars's gravity by means of the same vehicle. The MAV was only designed to reach LMO, and even the stripped down version fails to reach the required velocity.

I too, repeatedly asked why the HAB wasn't buried, and eventually concluded that this must be related to the short stay and the weight required for excavation equipment. Remember that the crew spends many times more time in space than time planned on the surface.