Because it never before had loot that dropped at varying levels. Thats why. This means even if you get a drop it might not be an upgrade. So if you got a drop at every boss then all you are doing is slowly giving yourself better chance of increasing your light. Its not game breaking, nor a complicated concept. If you got a drop every week from every boss, it won't be beneficial anyway unless the drop is higher than the one you have. Its still nice to get a drop though rather than moldering shards which don't even give hard mode loot.