I'm guessing because a Google search of "Adderall side effects" immediately returned this:

"Rare but severe side effects of Adderall can include emotional instability, psychiatric disorders and cardiovascular events. Adderall misuse can lead to seizures and death."

It's worth mentioning that people who take Adderall medicinally are taking substantially lower dosages than those who take it recreationally. Medicinal users take on average 20mg and can have dosages down to 5mg where a recreational user is taking 40mg and maybe up to 100mg. Sure, Adderall can cause the things you mentioned, but the same is actually true about sugar (a highly addictive substance, high potential for abuse, and limited medical usage).

These bins are arbitrary and dosage matters. It would be perfectly acceptable for the FAA to require 3 months of usage, at a specific dose, wherein a pilot is stable on their meds and an FAA doctor (which they must visit anyways) approves them. You can get a good idea of risk factors and stability by doing this.

There are rare but terrible side effects for virtually all medications. What is your point?

That you don't want people with suicidal ideations flying planes.

There are plenty of medicines that warn against operating motor vehicles or heavy machinery, off the top of my head some blood pressure meds like Losartan and HCTZ because they can make you dizzy/prone to blacking out. I imagine there's similar logic here for pilots.