The reason is that this is a default setting on the game to have the customs, and as such, they directly represent the quality of the product. They are an official part of the game since they are officially supported and default part of the experience. If it was unofficial, you could have any old shit (sanic, memes, etc. or crappy flame armour), because that's not a part of the base experience, and requires effort to activate and witness.
Here are 3-4 examples (I'd add screenies, but I have the customs off because of some bad ones):
- Imo, Aine and Alddras small pack/fur cloak, etc. skins are within the games current artistic style, are subtle, and fit within the world, and don't bring you out of the experience. They look like they naturally fit in the universe.
- BattleWizards backpack is decently drawn and done, but it's an object pretending to be 3D whilst obviously on a 2D mesh. So here the issue is more that the texture isn't awful, it's more it doesn't work with the model it's placed on, and looks unprofessional because of that. The fact it's a 1900 backpack on a character from the 1200-1600s isn't quite as off putting than a piece of paper trying to pretend it's a fully fledged 3 dimensional item. I'd say the same thing if it were wings or "spikes" drawn onto armour.
- Ohmygods customs are the ones I've heard the most murmurings about and the main reason i disabled customs, because it's just a lot of red thrown on top of the iron plate mail, and stands out like a sore thumb in the game. It lacks subtlety, it lacks much nuance and is distracting in the wrong sense. It doesn't catch my attention because it's interesting and beautiful, it catches my attention by assaulting me with it's, failing the find a better word, obnoxiousness. It just doesn't fit on any level. I argue this with anything not really stylistically considered.
Consider this for a new player. What's their reaction would be between ELs main server and the unofficial customs mirror if they trade it first. One looks like a game, the other looks a bit like a shit show at times. Same argument: Between an official valve L4D, TF2 or CS server and one of the unofficial meme servers, there is a difference in how you perceive those, and it'd be very strange if you had those memer servers as one of your default valve servers.
If i came in as a newbie and saw the most outrageous example of the current customs, I'd just think this is some shitty my-first-game or hobby project because that skin that i see as default, and as far as i know, is a part of the game as the whole, looks shit, and with the old graphics, I'd think it's just a random mod project.
I'm not attacking any of the players here, I don't really care about who they are, but the customs as a graphical element of the game on default NEEDS to have a level of quality. They just need to fit with, if boiled down:
- Would you use this for an item in it's own right?
- Does it fit stylistically with the world (Not obnoxious or the wrong art style)?
- Does it work at a basic level (if it's representing a flat object, is it flat, if it's 3D, does it look 3D, if it's attached to part of the body, is it attached)?