Whoever came over to the Dark One did so because of greed/jealousy/lust for power/immortality whatever. They were not the nicest people of the bunch. When they "traded their souls to the devil" to get whatever it was they THOUGH they wanted, there was no reason to ever WANT to go back. The base nature of these people was amplified through dark deeds/dark rewards/dark punishments.

They most likely are simply so arrogant they never ever THOUGHT that anyone would PURPOSELY kill themselves with something that gave exactly an HOUR of life left in order to get out of their oaths.

Think about it, the last HOUR is impossible to pinpoint in any case except for very specific poisions. Even grievous injury, you do not know when that patient has entered their last hour. They could spend days opening their mouths trying to say "I'M BLACK AJAH" and having it choke off until the last hour when they're most likely already so weak they can't say squat anyways.

I think it is not stupidity, just sheer arrogance which we know the forsaken and the dark one have in abundance. You join my side why in the WORLD would you EVER wanna renounce such?