It's just a skill issue on the part of the developers.
Making anti-cheat properly is hard. Writing a spyware that watches everything that happens on your PC and blocks any attempts of touching the game is way easier, but bypassing that is easy with solutions that have higher privledges, thus being invisible even for the anti-cheat. You can just fake calls or hide memory from the anti-cheat, or just edit the anti-cheat in itself.
The solution for that is to run anti-cheat in the highest possible permission - the kernel.
Now, you could just make another kernel-level program that would have the same permissions to defeat that, or just edit your OS (i.e Linux, or a VM) where your cheat lives outside and has even higher privileges than the anti-cheat.
This is where Windows comes in - the only way to run kernel code is to have it signed by Microsoft, and that certification process is extremely difficult and annoying, which puts a pretty big hurdle in front of cheat developers. It's the easy way out.
You could also somehow reverse-engineer Windows and run a custom version to bypass this. And that's where TPM comes in, which (if I understood it right) validates that your Windows is the official signed one, and thus the kernel anti-cheat is safe. You can't have this kind of affirmation on Linux, and the lazy developers who don't want to invest into actual moderation and proper anti-cheat solutions just resort to kernel anti-cheat rootkit and require TPM to be enabled.
There's not much Steam can do about this, aside from locking up their OS with signign keys and certification for priviliged software, along with setting up the whole TPM so you can't run modified versions, which isn't really possible since they are based on Linux.

Oh, cool, so if I understand it right, you have a hardware that directly reads the physical memory, so you can access it unrestricted and undetectable from another PC, where the cheat runs, and then you use a HDMI fuser to merge the output of the game and the cheat that runs on the second PC on a single monitor.
That's actually really clever, I love solutions like this. Not that I approve of cheating, I have 0 respect for people who (unconsesualy, as in all involved parties agree to it being allowed) cheat. But from the hardware/security point of view, it's amazing.