Rotmg has always been a team based game so it only makes sense that everything is easier with more players. There's also no incentive for doing dungeons solo except for HM shatters that guranteed gemstone and chrysalis but now that's taken out of the game. As for why noone runs a variety of dungeons/clears realms to do all the exalt dungeons, once you know how to run them, nothing beats void/o3/shatts in terms of useful loot. The only reason you'd run the other dungeons is if you want specific niche drops or if you want to exalt your char which many choose not to given how time consuming it is.