Pixels are (currently) the only phones that allow for all of the following at once:

• Proper verified boot
• Bootloader unlocking (this is most important for any custom ROM installation, regardless of ROM)
• Hardware memory tagging
• Full hardware isolation
• Hardware key attestation
• Ability to disable USB data (and also USB entirely) at the hardware level
• Everything else on this list

In short, it’s simply because Pixel currently has the most hardware level security features of any Android phone (on top of bootloader unlocking), for now. The Graphene team is allegedly in talks with an OEM to produce a phone specifically designed for it, which may be just as or even more secure. Time will tell.

I feel the need to mention that I’m not trying to shill for Graphene and especially not Google. Depending on your threat model and goal, Lineage or similar might be just fine for you. I just don’t think there’s anything more secure than GOS at the moment, and if that is important to you, along with minimizing bloat, it’s a great choice. I do highly recommend avoiding root and instead just get something that you can unlock the bootloader for, and then install a degoogled ROM. Just make sure you don’t accidentally buy a permanently locked phone, make sure it says unlocked somewhere in the listing.