Who needs orcs? Or elves? Or any demi-human?*
Could you make a fantasy world with only humans as the only humanoid race in it? Of course you could. Let me rephrase that: could you make a good fantasy world with humans as the only humanoid race in it?
You can…but there are a couple of things you would need to consider. It could also be rewarding, because it will force your players to make the characters special, rather than relying on a assumed, but never properly explored, racial culture. If I say my character am a dark elf, I am (probably) just relying on the assumed racial culture to carry my character concept, rather than my incredible and in-no-way-overstated powers of roleplaying.
Are human races / cultures different enough to justify mechanical changes?
Do the Northern People get +2 Constitution because they are toughened by living in a frozen wasteland? Or do you just use the rules for human characters in your game for all humans? If you use the former approach, you could effectively reskin the elves, dwarves, etc that are in most roleplaying games as different human race / cultures. I understand the need to want to do that, so that you do not “waste” anything from the roleplaying book that you paid your hard-earned money for. On the other hand, the later approach is easier to use at the table. In most roleplaying systems I have seen, the non-human races often excel in a particular area (such as being an archer that prances in the woods, or a sneaky thief), so if the game moves even slightly away from their strengths, the player may feel like their character is weak. However, humans are always a solid choice, with easy to understand mechanics. Even in a skill-heavy system like Pathfinder, the human bonus of +2 to one statistic, on extra feat, and extra skill points is easier to implement in game than the bonuses that other races have.
While I do not like “wasting” any gaming dollars, I would just leave humans as their usual generic selves and not try to create a bunch of human cultures / races with their own mechanical bonuses / penalties. If you want to spend the time, go for it, but I think it would be too much effort for not enough payoff.
What do you do with the demi-human enemies in your Monster Manuals, Fiend Folios, Archenemy Albums, Terrorist Tracts, Villain Volumes, Opponent Omnibuses, Detractor Dictionaries, etc?**
Do you simply ignore them (and not get the full value of your gaming dollar) or do you re-skin them? I definitely lean toward the latter option here. That orc? He is now a Viking. That hobgoblin over yonder is now a Roman legionnaire. Your players should not notice. In many cases, the race and racial abilities of a demi-human enemy is little more than window dressing. This reskinning of foes allow you to quickly have human opponents of varying abilities and power levels, without creating enemies from scratch. Let no one say that The Mixed GM didn’t try to save you a little time!
* The Alt-Right DM inspired this post with his excellent post found here.
** I went overboard coming up with these silly alliterations. But what is D&D without silly alliteration in their badguy books?