I don't think the situation is so clear-cut to be able to make categorical statements like that: server-side cheat prevention is also bypassable, and depending

I don't think the situation is so clear-cut to be able to make categorical statements like that: server-side cheat prevention is also bypassable, and depending on how it is implemented, also has the potential for negative side effects. No matter what approach the developer takes, they're going to be making a trade-off of efficacy, player convenience, and developer effort, and that trade-off is going to be influenced by a lot of different things, such that client-side cheat prevention (or, likely, a mix of client-side and server-side measures) will be the most reasonable option in at least some circumstances.