As mentioned in another comment there's no difference between someone choosing say a furry avatar vs someone choosing say a star trek character as their avatar. Same reason: they enjoy that world/universe.
Why do people make pop culture jokes? Because it's something they familiar with and enjoy and that gives 'em the seratonin.
It's probably tied to internet culture/subculture - VR Chat is still a pretty niche thing, most people don't own VR headsets. A lot of people owning a headset and interested in participating in a virtual world intersect with online subcultures and Otaku subculture.
Personally, I don't understand why there is so much surprise. If you really think about it, your avatar is something you will look at a lot. So you naturally choose something you like. And people have tendency to chose live objects rather than say, a gigantic 3D number, particle system, or white light. There is not really much choice; humans (humanoid characters, like elfs), animals (& imaginary creatures & hybrids, four-legged animals are typically weird to control in 3D environment) and maybe robots.
I feel seriously old since I've been using furry fox avatar in SecondLife back in the 2006, and even then, this was fairly common.
People with similar intrests tend to clump and early adopters of a technology tend not to represent the general population. Not to mention there are very obvious reasons why people drawn to roleplaying as something other than they are would be drawn to a technology that lets you easily do that in a relatively risk free environment.
I also wonder if its really as many as you think it is, or if they just stand out to you because you don't like them. You also don't have a good basis of comparison since you have no idea how many people in regular life are like that (i imagine its still much higher in vr space, but maybe the gap is not as vast as you think)
