Really? ZZTop's Billy Gibbons is an accomplished soloist. His melodic leads along with the rhythm section is what made them, not to mention his catchy riffs and the simple lyrics. I've listened to them as far back as Tres Hombres (1973)when they were still wearing cowboy hats. May be not the most technical accomplished band, but they have many catchy songs that are still on Jukeboxes.
I have been following Billy Gibbons a long time having grown up not far from Houston in the 60s where the Moving Sidewalks frequently played around. Hendrix had good things to say about Billy. Most rock guitarists improvise when they solo. I copy signature riffs but improvise extensively which is what the artists themselves do when they play live