

Nomadic people don’t just wander around aimlessly, and there are big differences in how desirable different territory is for nomadic hunter-gatherer humans. The principle is the same as with nomadic pastoralists: your group has a territory which can sustain them when hunted on/gathered from/grazed/etc over the course of the year, and your group will wander within that space in a deliberate pattern. If some other group decides to “just move on to” your group’s territory, hunting the animals and foraging the plants that your group knows they are going to need to survive the year, that’s an existential threat to you. And you can’t “just move on” yourself without wandering into the territory of yet more groups whose territory borders yours, and who will react violently to your presence for the same reasons.
Given the choice between fleeing to who knows where and fighting who knows who for the privilege of moving, or staying right where you are and fighting for the land you know your group can survive on, you stay and fight.
Humans spread out across the earth as the losers of these conflicts (those who survived, anyway) fled until they stumbled on new-to-humans territory, often displacing or eradicating groups of more “primitive” hominids they found there. This process continues until just about everywhere which humans can reach and which can support human life has humans in it. But expanding populations, the occasional natural disaster, and normal human frustration that their territory sucks while their neighbors have it great (which was often true; again, not all land is the same to a nomadic hunter/gatherer) meant that these conflicts were constantly reignited.
I’ll stand by the position that the Enterprise augment virus arc was an error, and the “explanation” for Klingon ridges is the same one you should use for the bridge of the Enterprise looking like it was cobbled together from plywood and plastic beads. This issue was best left to Worf’s lampshade in DS9 Trials & Tribleations.
It’s really interesting which visual differences humans will accept unthinkingly and which we will demand answers for. The Klingon ridges thing comes up constantly, but I have yet to see anyone earnestly ask why all the characters in Lower Decks have huge eyes and unnaturally uniform coloration, or why hand phaser beams in TOS go so much more slowly than later phasers and why everyone agrees to stay really still while they are being fired.