Because you get to use your brain instead of just dumping everything in the magical stash because of the need to evaluate what is worth keeping and what is to be left on the floor.
If it's anything like the inventories in the IE games, there's enough space to keep everything you might need so it's just a matter of dropping, or not picking up, junk. That doesn't require a brain, just tolerance for drudgery and pointless busywork.
If they dropped the main inventory altogether and only allowed equipment + quickslots, then there
would be a genuine challenge. More realistic too.