I mean, that's been hard for decades. It's mostly about the complexity of initializing the massive amounts of hardware.
You can certainly install Libreboot on many machines, but it's a relatively small hardware compatibility list-- thanks to the terrific amount of variation between computers out there.
Or there's a few manufacturers that will sell you a machine with CoreBoot onboard.
The BIOS is signed by a key, and the hash of the key is fused into the HW.
Anything 11th gen or newer locks libreboot or coreboot out.
Search for FPF OTP FUSE. It is an interesting topic. I want to drop intel ASAP due to this.
Not all OEMs are doing this. (And some are even shipping 11th generation with coreboot). But you're right in that it's making things worse.
Linux fwupdmgr can be used to verify the secure boot status and you will fail HS2 if this is not fused.
Also.... would be happy to know who doesn't fuse it on newer machines. And even coreboot does not mean you can flash your own. You can have a lockdown where you can only load the OEMs coreboot.