Because IF the dungeons dropped guns or you could control your drops better you would have statistically high chance of actually getting what you want and Bungie has special data analysts that know your playtime would then decrease. IF you got what you wanted from Dungeon, seasonal activities, raids and even PvP Iron Banner most players would stop playing because the actual playing is NOT FUN. I don't have an exact date, BUT at some point Bungie realized they were not making fun activities, many players were not having FUN in dungeons and raids but they slogged through hoping to get the rewards they wanted and a dopamine hit from a drop. The game was initially designed with the "Bungie 30 seconds of Fun" motto. In the old days Bungie realized players needed a splash of Fun about every 30 seconds and they wanted that loop to always happen. However at some point they realized it would be better to just capture players in player engagement metric numbers to show bosses "look we have players logging in every ___ days and spending at least ___ hours" and the goal was to keep increasing those numbers. The BEST way to increase those numbers was no longer the 30 seconds of Fun, but instead a loot treadmill, terrible content with too many armor drops and fail mechanics wiping teams keeping them stuck for hours at bosses just to still not get what they want so they log in the next day and spend more hours trying again and again. it's actually brilliant. BUT some players are now waking up and realizing it's no longer fun and they are quitting to play other fun games that are also more rewarding.