I have a question for you linguists and philos and biblios: why are tough and enough not pronounced like cough, and why are slouch and couch not pronounced like touch?

Is there a rule of some kind concerning the consonants before the vowels? I can’t think of a reason – or is it simply because they have derived from different languages, and have “retained” or modified their original pronunication (German vs Latin, for example)?

If there’s a simple rule I’d love to know it!

Thanks smarties.