Well, I have worked with loads of colleagues in engineering & finance who are very witty and funny. But funny people usually work in boring jobs, ime, and visual artists tend to be fairly boring people (mahoosive generalisation based on going to 2 art schools and being a painter for a few years).
Banksy is unusual in having the wit of your favourite colleages, and having a platform to display it in public.
Surely you cant have forgotten his boasting that he had a sculpture in the Tate Gallery? ("When we find Mr Banks's sculpture, we will return it to him")
maybe you have to have tried to break into the contemporary art scene to fully appreciate that.
BTW, I always judge grafitti on its artistic merit!