Because those are the phones that are supported in the upstream Android Open Source Project (AOSP), which these OSes are typically based on. Other phones, even ones that to a great job of publishing their sources (like Sony's), have their support living outside of AOSP. And older phones get dropped from AOSP, the original Pixel was dropped in Android 11. So, by only targeting the devices that AOSP supports these OSes can focus on the interesting part of building the OS, rather than getting bogged down with hardware support.
The other aspect to this is that you can install a custom OS on the Pixels and still re-locked the bootloader, which means you get Verified Boot and all the security guarantees that brings.
You actually couldn't do that with the original Pixel (which until recently, Android 11, these custom derivatives tended to support). You'd get a warning screen every boot about how the OS has been modified.
You definitely could, we used to support it in a previous iteration.
This was also possible on the Nexus devices, although the oldest I've tried it is the Nexus 6P.
It just worked slightly differently on those, nowadays you enroll the public key by flashing it to the device, on those (Pixel 1, Nexus) you used to have the public key embedded in the kernel.
Unless I'm mistaken, the Pixel 1 blindly accepts whatever pubkey is embedded in the kernel, but displays the warning screen on boot if it's not Google's pubkey (to clarify, not a click-through screen, just a temporary splash screen). I guess yeah it's technically Verified Boot, but if it just accepts any key you throw at it, then the security guarantees are a lot less. You can't tell it about your pubkey to get the scare screen to go away, and you can't tell it to block other keys to get the security guarantees.
Telling even the newer devices about your pubkey doesn't get the scare screen away. You see a Yellow Verified Boot warning meaning the OS is signed and verifies but with a custom set of keys.
When you lock the bootloader you block other keys, since fastboot is pretty much disabled when you do that, and the only way to install something would be via OTA updates which would have to be signed with your custom keys.
I guess maybe if you're able to get a root exploit and replace the boot image? Not exactly sure what would happen then, need to try.
