Why do they do this?

"tounge"

You'd be surprised by how many people spell it that way. That doesn't make it any less wrong, of course.

I wouldn't be surprised.
In fact, a few minutes later, I came across a user that spelled it wrong. It's always been one of those misspellings that jump out at me.
Does anyone else pronounce it "townj" in their head, or is it just me?

Indeed, yet it's so prevalent. My boyfriend also often spells rogue as rouge and for a split second I always think he's talking about something red before I realize he doesn't speak French.
Whenever I see "tounge" my mind thinks of it as tongue but with more "oh" around the middle part, like "town-gh". But I'm not a native speaker so my mind is probably a bit warped in that respect.