> Why do you need amphetamines to do this?

There's also atomoxetine, but it's not very effective.

> Why not a legal stimulant?

Which ones? People absolutely do self-medicate with coffee, ephedrine, or even cocaine where it's available (coca tea). And these stimulants do work, but they have _more_ side effects than amphetamines when used in theurapeutic doses.