Same reason people like to accessorize themselves!
Especially in social games like virtual worlds (now called metaverses, again), where the main activity is to walk (or fly) around and chat and show yourself. I worked on a virtual world called Blue Mars (build on CryEngine) over ten years ago that supported in-game developers, some of who created minigames but most of them made avatar accessories you could buy in-game. Blue Mars shut down long ago and all the developer doc I wrote on the wiki is gone but somehow there's still a page about avatar customization.
http://create.bluemars.com/wiki/index.php?title=Avatar_Customization