"They don't have to have dark skinned because (other world/etc.)" is Watsonian reasoning. Doylist, the writer just left them out when it'd be just as easy to include them, and the tendency to do so is an issue. If you live on earth odds are very, very high you know humans come in a lot of colors. If you do anything internationally, then you're aware there's going to be some variety in your audience.
And heck, the book's a book, it may not describe plenty of people, but there's always room for adaptations to represent. White = default's just kinda lazy.
When Humanity Fuck Yeah! Fiction and other types of science fiction say humans are different then other species it's often Means traits Western liberal Culture has. Like a technological progress fetish, hatred of the rules man, individualism and other shit. I thought about this while reading GURPs Uplift and it said Most humans don't like the idea of the Uplifting client system. Even do it highly resembles colonialism which you know was practiced for a few hundred years or even a few thousands years depending by how you define colonialism. But modern Western liberal culture doesn't like colonialism so their for all humans hate it.
It's worth noting that Humanity is literally the poorest species in multiple galaxies (who isn't under the control of a richer one), and only due to the friendly intervention of an ally did we avoid being placed under someone else's control, and often find ourselves at a disadvantage due to being the new kids on the block. So it's not
just that we have a reaction to colonialism itself, but that the Five Galaxies society is a ladder, and we're as low as you can get while being independent.
There's both the philosophical objection, but humanity's objection also has a lot on the practical end. Reinforced by some of the callousness shown by elders seen throughout the book showing it's not just an us-problem, dapient species upgraded in ways they don't like to be more 'useful,' tales of multiple client-species uprisings in the past, most put down, some, depressingly, succeeding but just taking up the old system with them on top. Etc..