I was just discussing this with a friend-According to Wikipedia the Federation is composed of " more than 150 member planets and thousands of colonies spread across 8,000 light years of the Milky Way Galaxy". In contrast, the enemies of the Federation are made up of one member species. So can someone explain why the Klingons, the Romulans, or the Cardassians are even a threat to the Federation, which has at its disposal the power and technology of so many different species working together?
Because it was never really thought out very clearly and didn't make a lick of sense, that's why. Moreover, it was fairly clear that, in TOS, we were watching the adventures of an Earth ship, not a Federation
ship. The difference being that there were untold different fleets out there as part of the Federation. Sadly, this angle was quashed in TNG and beyond, turning the Federation Starfleet into a 'homo sapiens only club.'
Consider how powerful the Vulcans and Andorians are during Enterprise: they have their own worlds, fleets, armies, etc. Did all of that really go away when the Federation was founded and they meekly retreated to their homeworlds so that the mighty humans could defend them in human-designed starships?
Please. Like much of the vaunted 'canon,' the organization and power structure of the Federation doesn't make any sense whatever and is a thinly-drawn conceit to focus the story on human characters, save for the token 'outsider.'