The "gu" combination is just tricky/rare in English, while the combination "ou" is common enough to be an attractor. See also "tounge" and "chaise lounge". "Lou

The "gu" combination is just tricky/rare in English, while the combination "ou" is common enough to be an attractor. See also "tounge" and "chaise lounge". "Lounge" and "rouge" also share the property of being legitimate spellchecker words in their own rights. We don't see this phenomenon happen with, say, "plauge" for "plague", because the diphthong "au" (as in "tau") is pronounced differently from long "a". But "ou" (as in "thorough") could be pronounced exactly the same as long "o". (And compare "restaraunt", as in "aunt".)