...because that's how the creators generally chose to portray it -- by giving it all of the characteristics we associate with states in real life. What few contradictions there are to that portrayal strike me as being quite minor.
Replace "states" with "humans", though, and you get the creators' way of portraying Vulcans, Klingons, Ferengi and other nonhumans.
Even when we get "essentially the old ho-hum with minor contradictions", I say we should concentrate on the contradictions and go for the imagination angle, and ignore the unfortunate fact that it's all built on today's reality. That's what the creators would want us to do...