I get that the reason is to make the show more familiar and relatable to a (primarily) American audience, but it comes at the expense of in-universe realism and makes Starfleet come across as being the sole domain of humanity.
Well, I dunno. In-universe it's perfectly believable for there to be uneven levels of political and military power between member worlds of the Federation and for one race to have hegemonic power, it's only a bit dubious for it to be a projected United States. That it complicates Starfleet's claim to being truly representative in ways that are never really faced up to is a given, but no moreso than American hegemony's claim to truly representing "the free world" is / was complicated. (It's only a shame that they never brought themselves to really exploit those complexities for stories.)