.../s.
Because parochialism.
Using soccer (any sport) as an example -
City a fans hate local City b fans when they play each other. When a or b play c (out of local area) a and b fans are against c. Proud to be from a+b.
If a, b, or c play against d (very far) then a, b, and c become United. Ie, North-South divide.
At a national level a-z are suddenly patriotic and suddenly everything is America's fault.
God, I hate team sports almost as much as the misuse of commas and religion :-)
