Yes i do realize that BUYING exists but that not the point here, The point being that fortune boxes aren't THAT easy to get and when you do the chance of getting epic is low, thus not very rewardind... they are a fixed prize on the free boxes and getting rare tier items out of them is not rewarding at all, grinding them on overlords den just to gain rare and SOMETIMES an epic is not rewarding enough.
[ Buying stuff is irrelevant here, and so is "other games out there" because it is about Super Mechs we're talking about. ]
Common drops are indeed rarer than legendary Bruh!