Years ago, around when the updating system was revamped an explanation for it was given but the gist was along these lines:- Your inventory is not stored locally and to prevent changes being made during synchronization, matchmaking and loading transitions you are removed from screens allowing it. If something, anything gets lost it's gone and cannot be recovered even by Bungie. Yes sure it's annoying sometimes but it's not going to change and personally I value not risking the loss of that god roll weapon, armour or 96 stat exotic that took many hours to acquire. I can set up before launch, make sure my quickslot is up to date or be patient till I land if necessary.