It's because the Pixel is unique in that it allows the bootloader to be relocked with a custom ROM so the same level of boot verification and other firmware level security can stay activated. No other phone lets you do that.

It is ironic that Google are the only ones who do though, yes.

GrapheneOS is great btw. I got a Pixel 6 Pro running it, my girlfriend (completely non-techie) has a Pixel 6a running it, neither of us has had a single issue using it.

I obviously did install it and set it up for my gf before I gave her the phone, but since then it's just worked like a normal Android phone.