I agree that this is the reason, but having Intel as the guard only makes it so that it could have already been hacked/leaked/bypassed and you never know.
At least if it was user controlled we can ensure that other people's leaked keys don't bypass our security.
If it's user controlled what stops an attacker from bypassing it as the "user"? Most people just want to have a secure device and will not think about security, not want to do any work to secure their device.