I think there are two reasons for that:

1. they don't want to release OS with lower security than their baseline
2. they don't want to spend effort on such "lite" OS support for many phones, and instead they choose to focus on their main builds

It should be possible to build a soft-fork of GOS for probably any device that supports e.g. LineageOS, but no one wants to do it.

I'm with you.

But it's a tough call on Graphene's part too - right now only us in-the-know have any awareness of the implications of Graphene's approach (security first) or even alternative OS for Android devices.

They probably think getting that out to the greater public is the crucial first step - get that accepted as a thing.

Also, by the time the general public is using Graphene devices, other phones may be shipping with the necessary hardware. Remember, most people are buying new phones every couple years when their old one starts misbehaving - to most people a phone is a toaster.