After I Finished the quest to wipe out the Dark brotherhood, I thought to myself "Maybe Skyrim will be better off if I wiped out the thieves guild too." So I ran into the Ragged Flaggon and began spamming master level destruction spells and such, only to find that everyone was marked essential. The elder scrolls wiki confirmed my suspisions. There was no quest to wipe them out. Why not? Why can I not Cleanse Skyrim of their filth with Sacred Fire?

Thieves Guild didn't have the same writer as the DB. Some quest writers only think linearly.

Because they're not typically evil?

It's not as if the Thieves Guild in Oblivion was considered 'evil', either, nor was it a filthy scourge. The Dark Brotherhood are killers, who just so happen to relish in killing people. The Thieves Guild don't condone killing, at all. There is only one point in the quest where you're sent to actually take someone out, and it's a member of the guild.

Because the thieves are not murderers, so they cannot be murdered :shrug:

I dunno, perhaps becuase rats, as vile as they are still serve a purpose like we all do?

The Thieves Guild have connections everywhere, that's why. Think Shadow Broker in Mass Effect.

well you get to kill the shadow broker in mass effect.

i was disappointed that you could not wipe out the thieves guild as well.