Because getting all HUMANS to agree on a particular way of doing things took hundreds of years and a nuclear holocaust that resulted in quite a number of them being entirely marginalized (on a world with a billion and a half Chinese humans I can think of maybe five Starfleet officers of Asian let alone Chinese ancestry).Why not?
Compatibility is enough to cooperate without fighting a war, and under proper conditions is enough to trade, to meet and negotiate, to make mutual decisions about jurisdiction, science, technology and galactic politics; in other words, it's enough to debate things in the Federation Council. It is NOT enough to completely whitewash the entire cultural history of those races if they ever want to do anything meaningful in space. This tells me that many of the 150 Federation member races probably DO have their own Starfleets that, while under Federation control like "our" Starfleet, doesn't have an academy or a headquarters in San Francisco and doesn't give its ships names like "Enterprise" and "Crazy Horse" in big English letters with Arab numerals as identifiers.If there are 1000s of other races that they have met, 150 sounds reasonable enough when it comes to "compatibility" with human ideas.
Actually I think something like this was hinted at in Unification where the Vulcans seem to have their own ship designs and, possibly, their own fleet. This would make sense in light of the condition in ENT, where the Vulcans have their own starships that are already technologically superior to anything Earth has ever built. It's very likely the Vulcans and even the Andorians BOTH continued to operate their own fleets and their own ship designs well into the 24th century.
Why not? Everyone else tollerated the Tellarites, the Vulcans, even the Andorians (who in a number of ways are as bad as the Klingons). Apparently the Horta eventually joined the Federation too. I really don't see the Federation as the "Homo sapiens only club" lampooned by Azetbur; if it is a multi-cultural organization, then the diversity of its members really would be a major obstacle in its cohesion--as we saw, famously, in TOS "Babel"--which is part of what necessitates the need of the Federation in the first place, for the sake of galactic peace and cooperation.I doubt Klingons will ever be members, or Sheliak, or Tholians, etc..
But again, cooperation is not acculturation. Starfleet exhibits alot of behaviors that reflect distinctly Earth attitudes and Earth cultures, including similar rank schemes, organization, tradition, technology and protocols and so on. Which is fine, I have no problem with this; what I have a problem with is expecting that 150 totally alien worlds would submit to the cultural and technological domination of the entire quadrant by ONE of its members. "Your culture will adapt to service us" is supposed to be the catch phrase of the bad guys, remember?