Because Damon Albarn only has to perform a voice in Gorillaz concerts (it’s a cartoon, dontcha know -and therfore, I guess, there isn’t much improvisation, he sometimes performs in his underwear: “We’re not playing behind a curtain now, but it was great fun when we did. I used to play in my underpants sometimes.”
And thus Albarn completes the metamorphoses from cool to twat.