Thats still not the same thing as building something in the editor window as it requires the seperate asset window. If I build a group of Actors I like in the level window I have to copy/paste or recreate them one by one in a BP rather than simply being able to create a prefab. BPs have limitations too because of the construction script, something a prefab doesnt necessarily need as its a user defined data object (like a template) and probably doesnt need per instance settings either.
Again what you described is a round about way to achieve the same thing but it is not the same thing and doesnt have the same benefits as it requires very specific implementation planned from the outset. If you had read my earlier post it might have saved you explaining it.