Scrolls and unique items are largely passive rewards, though. Lords get some just for being lords, adventurers get some just for hunting or gathering. Every now and then they get the option to create one, but that's still fairly passive: either you have the ingredients or you don't, the option won't stick around for you to get anything you lack. Then, high-ranking nobles are pretty much automatically targeted, and many adventurers just offer it for free or little, there's limited negotiation involved.
So while yea, it's true that a lot of work was needed in the background, most of it would have gotten done anyways, whereas with infiltrators, training your infiltration ability really just goes to one end, else you are wasting your time.
Gabanus rises a point, though. The risk to a failed (or partially failed) assassination attempt is a ban. Magic has no such risk, at most it might backfire I believe. Perhaps a chance of a possibility to place a ban on hostile casters would be legitimate.