They actually don't. They create a short term boost to certain aspects of cognitive functioning. While, in theory, this improves performance, in practice, it is often bottlenecked elsewhere.

It's like giving a rally car more engine power. Without the ability to get that power to the wheels, it's, at best, useless, at worst counterproductive. It might feel like you have more speed, but your lap times will be worse.

On top of this, you have the body's homeostasis systems. In a normal brain, they will correct for the effects of the stimulant. This pulls the system back to its original state, leading to things like caffeine addiction. In the ADHD brain, their homeostasis systems are tapped out, they can't get the brain to an optimal state. The stimulant provides an artificial push, that takes a load off of other corrective systems.

It's akin to glasses. Someone can learn to see through glasses, when they don't need them. However, someone who does need them will gain a permanent improvement from them.