Youcan already get assasins/Ice Druids/Wind Mages/Lore Masters/Necromancers etc. through luck, you already can get magic items (like Robe of the Archmagi
) and fortresses (Fortified Citadel with order, and some kind of a castle) through luck...
Events can also create new sites, including gold- and resource-producing mines. I don't think they currently add unit-producing sites, but I think that would be possible even in DomII.
I think the real reason is that you can only get 3 events/turn, max, whichever scales you choose.