Unfortunately (or fortunately depending on what side of the fence you live), boot chain security is not taken as seriously in the PC ecosystem as it is on phones. As as a result, even if you relying on os features, you cannot trust them. This is doubly the case in situations where the user owns the kernel (eg Linux) or hypervisor. Attestation would work, but the number of users that you could probably successfully attest are on on a trustworthy setup is fairly small, so it's not really a realistic option. And that is why they must reach for other options. Keep in mind that even if it's not foolproof, if it reduces the number of cheaters by a statistically significant amount, it's worthwhile.

I really thought this might change over time given strong desire for useful attestation by major actors like banks and media companies, but apparently they cannot exert the same level of influence on the PC industry as they have on the mobile industry.