They're not, it's just the current rules of English.
It did used to be the correct form, but it's a bit old-fashioned now, and certainly no longer set in stone to the extent that it's incorrect to use anything else. And language changes because of the way people use it, so to a small extent, I can change the language by the words I use. (I'm not frivilous about this - one of my hats is "editor", so I do know the blacks and whites and greys of current English usage.)

Why should it be "correct" to spell 'lose' and 'loose' differently?
Cos they're different words with different meanings and different pronunciation? ;-)