Hi Pokket, thanks for responding.
There are a couple of reasons:
1.) The behavior is inconsistent with how all other containers work.
2.) In A19 exp. I've seen it where pressing 'R' to take all doesn't properly put the player armor back on which means with a full inventory you're stuck with items left in the opened backpack which then involves juggling stuff out of your inventory into the backpack so you can put your clothes/armor back on to get those slots back for other stuff you had when you died.
Please let me know how I can further help.