Humanity has such rich variety that, culturally, ones choice of a human PC enables you to play any variation - human fighter could be an honourable samurai, a rapacious Viking, a proud Zulu, a nobly spiritual Apache, a devout Knight..and so on. Where the Demi-humans come in is to identify a personal trait to the race as a whole and thus give you a clue as to the type of person you want to be in-game. Which is a handy starting point.
Personally, I'm tall and quite grumpy in real life, so I like to play Gnomes.
I've never played an Elf as I grew up on the books rather than the films so for me Samwise was more relatable than Legolas. Those who've grown up film first would probably be the opposite way inclined.
I want to be the hero, so playing a freak like a half-orc or Dragonborn (or especially a Tiefling) leaves me cold - it's all very well being the brooding, moody teenager, erm I mean ranger, but ultimately Aragorn becomes king - part of the establishment. And at the heart of most fantasy is the notion of acceptance (becoming king, or husband, or hero), or of vindication (which amounts to the same thing, only with two fingers attached) - and with that comes the concept of wanting "in" rather than choosing to remain outside of society.
Other posters have commented on the nature of dwarves and Halflings as being essentially human with lower roofs - semi-human rather than Demi-human, if you like. I've played one dwarf and one half-elf, with their defining characteristics being that they were, respectively, Chaotic Neutral and female. Both departures, but neither terribly racial - at heart, I played them as human.
Everyone else has been either a human or a gnome. And in 5e the feat variant humans are pretty sweet.
But, each to their own - as someone once said, we should strive not for the homogeneity of the melting pot, but the collaborative individuality of the salad bowl.
Gnomes are cool, though, and I'll fight anyone who says otherwise #groinheadbutt #yourkneecapsaremine