Visibility check of what?
- The player and their shadow and all visible effects on the game world -> congratulations, now the server needs a GPU per player.
- The player's geometry? -> shadows pop into existence when the player's arm appears around the corner, and the server is still way more expensive than it would be
- A volume around the player? -> Still allows a significant advantage, still requires significantly more horsepower, and the client still can't do spatial audio
This amounts to making players use thin clients and putting all visual and audio rendering on the server if you want it to work and not suck. Will you be happy to save £1000 on your PC at the cost of having games cost £150 a pop? Thought not. Or did you think the "extra CPU cycles" were just free?