> there is very little evidence to suggest that humanity has any really significant colonies anywhere, and quite a bit of evidence against it
It is kind of suspicious that most of the characters are from Earth; Tasha Yar is from a colony but there aren't any other major examples.
> combined with the way the Enterprise in all its incarnations is always the only ship in the sector
Space is big, and sparse. There are apparently hundreds of planets in the Federation (explicitly stated at times). There are also hundreds of starbases. And while in the original series there are only about a dozen ships in the same class as the Enterprise, by the time DS9 rolls around we see huge fleets of hundreds of ships on screen.
It is kind of suspicious that most of the characters are from Earth; Tasha Yar is from a colony but there aren't any other major examples.
> combined with the way the Enterprise in all its incarnations is always the only ship in the sector
Space is big, and sparse. There are apparently hundreds of planets in the Federation (explicitly stated at times). There are also hundreds of starbases. And while in the original series there are only about a dozen ships in the same class as the Enterprise, by the time DS9 rolls around we see huge fleets of hundreds of ships on screen.