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.
If say 40% of all quarterbacks were named James, you are not explaining that by saying "James is a perfectly normal name, just like Richard or John".
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.
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)