> The server also doesen't need to tell you they are behind it if they're sneaking

This requires the server to calculate line of sight checks for every player, which is costly, requires loading the entire geometry into the server and would be horribly prone to latency. Then you're looking at potential performance problems on the client due to only knowing about a player the second its in view and having to stream the assets to the GPU, which if don't happen in time for the frame you'll experience as hitching.

> You already know where an enemy is if you hear them behind the wall

Yes but this requires using your brain rather than just seeing them straight up through a wall.