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.
under the doctrine that software "trust" is needed YOU are the attacker. It's entirely about stripping your control (thus ownership) from the hardware you paid for (see the safetynet shitshow).
There's a second use whereby I somehow bind my own OS hash to my own data encryption key, so nobody who changes the OS can read the data. The technical distinction between this and the previous: if it's designed for the device owner's protection, the device owner can reset the system.
Just like with HTTPS, you can enrol your own keys in the TPM module, or sign your binaries with a key thats already trusted by your system.
This is just establishing chain of trust, and does not prevent you from doing anything on your system.
True, this could be hypothetically extended to disallow booting third party binaries, but I would say that's just extrapolation for now and not reality.
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.