Well, you wouldn't force everyone to dress the same way, because a few people are concealing weapons under baggy clothing.
If a few people are using graphical modifications, it's not completely ruining the game for everyone, although it might make it a bit harder for new players, most of the mods aren't game breaking.
Those that are, such as transparent walls, are easily detected. They can be demo'd tracing people through walls, making it quite obvious as too who they are.
Making people more obvious isn't that much of an advantage anyway, as kill cam essentially renders it pointless.