Like most things, there is some nuance to the discussion that gets glossed over. There is a world of difference between "why even have species, they are just humans in hats" and "I want to play a half-ooze feyborn giantkin". Most players* will try to find a middle ground between those, and that...
It's ok. You're not supposed to. Everyone is different. I look at it the same way when people argue that X shouldn't be a class. You (editorial) may not see the utility in a separate barbarian class, but lots of other people do.
It comes back to the fact that playing Vampire (or Cyberpunk or Star Wars) the setting is in the rulebook. It's really hard to design a character that doesn't either fit with the setting unless you are intentionally trying to be contrarian. But D&D? The fact it has no single setting but a dozen...
Because people want them to?
This isn't an intricate delve into the human psyche. People want to play a pretty elf or a stubborn dwarf or a edgy tiefling. They want to look different, have a few cool powers, and be something a little different from what they see and do everyday.
They want to...
I still think that's a tall order for a couple of people sitting at a table eating snacks and rolling dice. An elf has all the basic thoughts and emotions of a human. They love, laugh, fear, cry, ponder, doubt and hate the same as a human because they are being played by a human. If you wanted...
A setting can be more than one of those. For example, Eberron supports heroic, mystery, intrigue, urban, war, and a smidge of science. I would hope a new setting would try to check off more than one box.
I play a human in real life. Most games I play offer human as the default choice. Sometimes I've want to play a blue tiefling or an angel person or a cat person because that's a story I don't get to tell everyday.
It was always supposed to be that demihumans had benefits but drawbacks. The biggest one was supposed to be level limits and class restrictions. But the problem was level limits only applied if you got to higher level and multiclassing more than made up for the limitations. Couple that with more...
"Your fanged bite is a natural weapon, which counts as a simple melee weapon with which you are proficienct".
Dhampir as it was originally printed. Thankfully it was errata'd in Book of Hungers.
I think the issue is +1 thunder gauntlets aren't a real magic weapon anyone can pick up and use. It's like how certain species had natural attacks that counted as weapons. You couldn't make +1 teeth for example.
That Said, I'm fine with that being a house rule. It's within the spirit to allow it.