Bungie paid attention to how random matchmaking for PvE content went in WoW and learned the lessons of it well. For some history:
-In 2008, the Wrath of the Lich King expansion was released, and PvE content (both 5-mans and raids) was drastically easier than it was in the previous expansion, The Burning Crusade.
-In 2009, along with the final tier of Lich King content before the next expansion, Blizzard released the dungeon finder tool, allowing players to queue up for any dungeon, be matched with four other players by the computer, and teleport straight there once the group was ready. Since these dungeons were all pretty easy (well, the new dungeons in 3.3 were mildly challenging) you pretty much steamrolled the place without having to plan or coordinate anything, got your loot/badges, and left.
-In 2010, Blizzard released the Cataclysm expansion. In response to complaints that LK dungeons were way too easy, Cataclysm dungeons were significantly harder than LK dungeons, pretty much at the level of BC dungeons. This meant that communication, coordination, use of crowd control, and tight mana management on the part of the healers was required to complete them, which the Lich King generation (referred to by many elitists as "Wrath babies") didn't really know how to do. This resulted in a lot of pain and suffering in dungeon finder groups and a lot of complaining on the forums until they nerfed the dungeons to be closer to LK level again.
-Ever since then, the hardest content in the game has not used automated matchmaking and requires pre-formed groups, and most other MMOs have followed that example. There are tools that allow you to find random strangers to group with on your own, but those are still pre-formed groups with a leader who has the ability to set their own standards for who can and can't be in as well as (hopefully) experience and a plan.
tl;dr- random matchmaking does not work with content that requires any significant amount of coordination or skill.
It is a fair point that there aren't many good tools in Destiny to form a group with strangers in town, though.