I've never understood this argument. I regularly run insmod and/or modprobe to load kernel modules.

I'm guessing it's because Linux constantly breaks ABI for out-of-tree kernel modules. And because every version of a distro can have a different kernel it's a nightmare of a support matrix.

That and Secure Boot establishing a chain of trust. Valorant requires Secure Boot and TPM 2.0 which means users can't modify the kernel driver. I'm not sure if Valorant wants to distribute their own Secure Boot keys and sign all the distro kernels + their module.

Because companies choose not to support Linux. Some popular ACs like EasyAntiCheat support Linux, but game developers (e.g. Facepunch/Rust) choose to block it.

Sure, the Linux kernel will let you load any unsigned kernel modules you want...but cheating is still possible (and happens) on Windows, so...

(Granted, trying to stop someone from running code on their own computer is a losing battle/stupid idea from the get-go)

It's not hard to build. It's just very hard to guarantee it won't be tampered with, making it ineffective.