Shaddy the guy
Professional ass-merchant
I think the main issue is that Sonic and Eggman's designs are inspired, at least in part, by older cartoon eras. Sonic resembles Mickey Mouse or Felix the cat quite a bit, and Eggman's rotund, simplistic classic design evokes a lot of that too, but no other humans have made an attempt at this. I think if you were to grab rubber-hose era humans (which there aren't that many of, and a lot of what there is is racial caricatures) and...do to them as Sonic did to Mickey or Felix (not sure how to word this), you'd end up with more characters that fit the style of the classic series better. From there, modernizing them would follow the same course as Sonic or Eggman. More detail, sharper angles, et cetera.