Why are all the aliens human?

I’m relatively new to the Star Trek fandom and I have been enjoying a good chunk of it, but there’s an issue I have with the alien populace.

Why is it that so many aliens are just humans with really small differences?

In both Enterprise & TNG (and to be honest all of Star Trek), there are so many human aliens with really small details to mark them as alien. Small nose ridges, strange markings, extra padding on the forehead. And so many aliens in TNG are just humans with a different name and nothing else. Even the Borg, the cyber race that has conquered and assimilated an untold number of species, are also just humans.

What’s up with that?