often seen in the street without a hat

Me too! My guess is that the vowels are wishy-washy, liquidy affairs, that need the support of all those fricatives and plosives (just feel the testosterone!) to keep them steady, and to contain them safely within their rigid outer casing of the word. There they can chat to other vowels and hold diphthong mornings, while the beaky prows of the the consonants carry the word through the adventurous pack-ice of Meaning itself.