Is the dungeon a random one or is it using someone’s key? How does it choose which person’s key to use? What if they don’t want to use their key? Do you queue for a specific key level and dungeon or a range? Does the system try to use 5 different classes for your group or just look at roles? Would the system balance out the party comp depending on the affixes (eg not have 3 melee on quaking, getting a soothe for raging, having Mass Dispel for bursting, etc)? Does the system prioritize making sure every group has heroism? How would you determine the minimum rating and gear breakdown for every key level and dungeon? Do you keep people in gear brackets or mix highly geared players with low ones assuming they meet the minimum requirement?
TLDR: Building an LFD tool for m+ would be way more complex than just having people make their own keys and isn’t necessary.