Flagrance or Ignorance?

Why the hell is it that no one can tell the difference between "then" and "than"?

It makes me want to strangle people when I see this malapropism in the writing of otherwise intelligent people. I must however admit that the other day while revising something of mine, I noticed that I had carelessly used "no" instead of "know". Not wanting to be a hypocrite, I almost didn't survive strangling myself.