You might think so, but it's notable that this is actually one area where it doesn't apply. Cover to creatures increases their Armour Class. (Other creatures typically provide a +2 AC).
Looking back through the public playtest documents, it seems that this has been the case for a very long time. I thought cover was handled by disadvantage at one point, but I may be mistaken.
Cheers!
I imagine , since missiles in melee is so common, and central to most non-frontline characters, it might be tedious rolling disadvantage that much. I'm all for streamlining the advantage mechanic frequency, so this simple AC adjustment is fine by me. It also probably stacks (or works in concert as a minus to hit) with other factors such as fog, heavy winds, and the like.
It also maintains an at-range penalty when, for instance firing at a blurred target, which will induce disadvantage in addition to the AC adjustment.
cheers to you as well,
Michael