What is your experience with a manual shift car? Have you ever driven one? Ridden in one and watched the driver's feet closely?
IF the car had a huge engine, like 10 liters and 500 hp in a Civic body you could simply release the clutch as fast as you want and the car won't stall. It would lurch ahead and perhaps snap you neck; but it won't stall. Such a motor would be too big, too heavy, and too expensive for a normal car.
The purpose of the transmission is to allow a small motor to move a big vehicle. Using lower gears to get it rolling and then higher gears to add momentum and eventually reach cruising speed. At the same time changing gears keeps the motor in a "sweet spot" of rpm's for producing power and torque.
The clutch is released slowly in order to keep the car from lurching, bucking, and snapping the driver and occupants around in their seats. Gas is applied in the process because a motor at idle produces about 1/100th of its peak power and torque. You need to have the motor spinning faster to produce enough power to move the car. Gentle starts on level pavement require only a little press on the gas. Fast starts, rapid acceleration, or starting on a steep incline require a much bigger press on the gas petal, and even smoother release of the clutch to accomplish without whipping the occupants around or producing a lot of buring rubber from the drive wheels spinning out.