I think most of these companies do do the server side properly. There are plenty of hacks that just make a client play ungodly well. Like macros, aimbots, cooldown tracking, auto-hex
GTA V is an exception because it's so easy to cheat in. I believe it's peer-to-peer with no verification among peers that what happened should actually have happened. It's basically impossible to secure that.
I suppose that was an intentional choice, I can imagine running the amount of worlds that GTA has (iirc you only have up to 32 or so players in a world? Something like that) doesn't scale well cost-wise. IDK if AWS and co were up for the task yet back when. But since you earn in-game currency, not having a central authority check these things is... an interesting choice.
I suspect GTA VI may improve on these things and have centralised/dedicated/anti-cheat-guaranteed servers. Then again, it never impacted their profit margins so idk.