ok this will be my first time trying to answer one of these, but here it goes.

When the engine of your car is running, it really shouldn't be stopped (stalled) unless you are shutting off the car. If you were to directly connect the engine to the wheels (like you do in a manual) and the wheels are not moving, then the engine will stall. That is why you need to give it some gas from a stand still while you release the clutch.

If you were trying to remove the need for a clutch, then you need to find a way to make the engine turn the wheels, but also not be directly connected to them so the engine doesn't stall at a stop. The way this is done is by using a torque converter. It uses liquid to transfer power from the engine to the tires. When your car is stopped, the liquid allows the engine to turn freely and prevents stalling.

This is why when you take your foot off of the brake your car moves forward (if it is an automatic). Although at 5 years old, I'm surprised you can reach the brakes.

Now this liquid energy transfer is not as efficient as a direct connection. Therefore when you use a manual, you're getting better efficiency and thus better gas mileage.

As far as being designed around the issue, they have some things they can do. Torque converters are becoming more and more efficient, and some even have a direct connection that can be applied at highway speeds to give you better mileage since the tires won't be stopping at that speed and the threat of stalling is removed.