Because, those drugs are actually, you know, fun to use?

They directly effect your serotonin receptors and release a shit ton of it over a short period of time. MDMA, Speed (pure amphetamine or otherwise), Meth, MDA, pseudoephedrine, cocaine, THOSE are recreational drugs. Adderall is no where near on the same level as those drugs.

Hell, you have to take several doses of it to get to the threshold, the other drugs mentioned act on even very small amounts.