Oh don’t be so chippy Capy.
For a start, I live here so United and City mean two clubs only. No one says “Man U” by the way.
For seconds, all the other “Uniteds” are from towns with only one football club, with the single exception of Sheffield. Who I’m pretty sure no one would have thought were a founding father of the PL from my post.
Those clubs are usually referred to be their town’s name eg Rotherham. No one calls Manchester United “Manchester” except some Americans and even then it’s becoming less common since the rise of City (yes, Manchester City).
Calling them United also winds up Newcastle, Leeds and West Ham fans. What’s not to like?