Why does the bottom part of an aircraft nose curve upwards?
Having the tip of the nose in the centre gives best combination of aerodynamic performance, weight and internal volume. If you move the tip down, the slope above it will either be steeper, leading to higher drag, or longer, leading to greater length and thus weight; or you don't extend the plane, but then the longer sloped part will provide less internal volume. Only the cockpit windows, which need to be on the sloping part, shift the nose a bit down, to the centre of the cross-section just before cockpit.

Cars are different. For a car you want the bottom surface flat, because you don't want to push and compress air in the gap between the vehicle and the ground. But aircraft has air all around and it is most efficient for it to push air equally to all sides, so a circularly symmetrical shape works best.

Also note that lift is yet another thing. Lift requires a sharp, downward-sloped trailing edge.