Because the vowel sound in “lose” is more commonly spelt “oo” as in “loose”, and “lose” looks like it should rhyme with “nose” or “close” but doesn’t. And “loos

Because the vowel sound in “lose” is more commonly spelt “oo” as in “loose”, and “lose” looks like it should rhyme with “nose” or “close” but doesn’t. And “loose” is a real world (albeit with a different meaning and pronunciation to “lose”) so people’s brains (and spell checker) don’t instinctively flag it as wrong.