Surely one of the great cosmic questions of our time.
Because “w” actually arose first as the digraph “uu”, while “m” didn’t arise from “nn”, but rather came from the Greek letter mu.
And, yes, in case you’re about to ask, “u” and “v” were once the same letter.
And then they separated them into two letters that are almost identical. That drives me nuts. N-V-T-S, nuts.
Shouldn’t that be, “That driues me nuts” or “THAT DRIVES ME NVTS”?
Arr! It’s driuin’ me nvts!
[Off topic] Ya know, my brother just heard that joke about the pirate and the steering wheel for the first time yesterday. He’d honestly never heard it before. [/off topic]
Interesting. Then why do we call a w a double-u instead of calling u a half-wuh?
And nice username/post combo 
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Because “w” actually arose first as the digraph “uu”, while “m” didn’t arise from “nn”, but rather came from the Greek letter mu.
QUOTE]
Doesn’t that just push the question back? Why didn’t the Greeks call mu “double-nu”? 
You want to really blow your mind? The lowercase Nu looks just like our v.
Because they were always separate letters in Greek, derived from separate letters in the Phoenician alphabet, which were derived from separate Egyptian characters. At no time was “NN” written to mean “M”.
I always drive my son crazy with insisting that w is sometimes a vowel - the example of this is the word ‘cwm’ (a mountain bowl-shaped valley). It makes sense though, because it probably should have ended up spelled ‘cuum’ (pronounced like koom).
And part of it is that the “M” sound is found in most languages, while the “W” sound is not found in a lot of languages – so when scribes from languages that lack a “W” sound have to deal with it, they improvise. One example is the French word “ouest”, derived from the same Germanic source as our word “west” – they write “ou” to get as close as they can to the “w” sound.
Not nu-nu?
Isn’t that a Welsh word? They are crazy about their W’s.
On “w” as a vowel – it appears in English as part of two-letter vowels, i.e., “aw”, “ew” and “ow”. In particular, in the word “ewe”, it’s hard to analyse the “w” as a consonant or even a semi-vowel, since the word has the same pronunciation as “you” and as the name of the letter U.
Yup - the other words like cwm from the Welsh is cwr (train tracks made from single long rails?) and cwtch (a cupboard).
Part of what drove my son crazy was that I used ‘cwm’ as a word when we were playing hangman… (that’ll teach him to doubt me about vowels - he wasn’t happy when he went through aeiouy and didn’t get any letters).
My last name (Davies) is uncommon except in Wales - my granddad left Wales as a youth (orphaned at 9, came to America alone at age 11…)
Ewythr Elc is Welsh for “Uncle Moose.”
[also off topic]
I first heard that joke from a very inebriated friend. He built up to the punch line by grabbing the invisible ship wheel that was tied to his junk, mimicked moving it back and forth, and exclaimed in his saltiest, scurviest voice, “Arrgh! I’M A PIRATE!”
I still think his version is funnier.
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