[travisgriggs]

On an approach, you are flying dangerously slowly (necessarily). You’re right next to stall speed. You want to go slower slower slower right up to the point you don’t go too slow. You want to reserve that crossing the threshold of too slow until your poised right over the runway with inches between you and it.

When you stall, you start falling at the speed gravity pulls you minus any drag your airframe presents. And if you’re already close to the airfield, you might be only a few hundred feet up, so you’re out of room to put the nose down and throttle up to regain speed necessary to regain lift.

Putting gear down adds a little drag (and a lot of noise), so a minor speed in reduction; going full flaps slows you a lot. You usually pitch the nose down a little more to increase your rate of descent as you go full flaps, so that you keep the speed up to keep the lift up which keeps your plane up. If it’s dark, you’re tired, flying close to stall speed already, go full flap without realizing you just did and don’t keep your eyes glued to the air speed indicator, you’ll stall out and fall from the sky. Trying to recover would catch a lot of disoriented pilots unawares.