PC Gaming

but why can’t server side cheat detection be the main focus?

Ideally, this would be the BEST way to prevent cheating, and it's the approach Valve has essentially. It's how they managed to absolutely obliterate every bot that was plaguing Team Fortress 2 for years and they haven't resurfaced since in a large enough group to matter, and any botnet that goes online quickly gets whacked. But the reason it's not focused on is because it costs a lot in terms of server overhead, and triple A gaming companies are more beholden to shareholders and cutting costs than any other company, so... They'd rather not spend that much on server stuff and offload the anti-cheat to the end user, which I find to be stupid. And also, a lot of companies have a vested interest on keeping cheaters playing or re-buying the game, like Tarkov, which is plagued with cheaters constantly despite having a stupid high barrier of entry, and my only theory is that there's so many russian hackers playing that if they banned them all they'd lose a decent enough chunk of the playerbase that it would threaten it.