Aside from the obvious that they want to make sure people aren't in matchmaking forever - the main reason they wont is for farming. PS4 - clock adjusting to spawn solo aside. Part of the reason they didn't add missions selectable from the map for replaying - (originally meditations; now daily heroics) was to remove farming. Back in D1 certain quest objectives could be farmed by doing an activity solo and essentially trivialize the quest step. This way, outside of nightfalls (where you can solo - but it has to be in rotation), even if there is a decent method to farm something - outside of you having a full fire team; randoms completing strikes as normal will pull you forward.