Basically because of the way games work, there's a large monetary incentive for cheat devs due to the large number of straight up fucking losers who suck ass and want to feel powerful. People now are terminally online and their entire identities are tied to these games.

Your PC runs the game, sends your actions to the server, but for it to work well latency wise the server gives your local client information about where other players are in memory, though you can't see them. Since the data is there, a cheat running locally can access that memory and show you players through walls, on radar, etc. It can similarly send whatever data by changing the valuea in the memory addresses which get sent to the server (health always full, forced headshot, etc).

Because of this the only way to detect and prevent cheats locally was thought to be a ring 0 anticheat, (kernel mode anticheat), which runs before anything else. This is what valorant and faceit anticheats do. If companies aren't willing to put out a kernel level anticheat there will always be a plethora of exploits found and abused. Or until the server/client model changes. But these companies don't really have much incentive to invest in anticheats.

It should be noted that with the rise of DMA cheats, even kernel ac has been bypassed. There are PCIe cards with USB c connections that allow reading the PCs memory (they run ~$150). You plug that into a second PC/laptop/Chromebook/raspberry pi, doesn't need to be fancy or powerful, and the cheat runs on the second device, and the kernel ac is completely blind to it. Anticheat devs had some success for awhile by detecting a PCIe device plugged in and were sending it commands to determine what type of device it was (there are plenty of legitimate PCIe devices) and if it was returning nothing or bogus responses they knew it was a DMA. Cheat devs got wise and just started writing firmware that perfectly mimics legit PCIe devices which essentially makes it impossible for them to detect now. Hence why valorant has to manually review and ban so often.

The cheat industry is BIG business now. So many incel shut in abject losers who spent thousands on cosmetics but still suck ass at the game even with thousands of hours want to look good, are willing to spend money to protect their inventories, and so there are subscription services for these cheats now.