When building products to last, it’s not enough to design the hardware to be repairable, upgradeable, and customizable. The overall longevity of devices as complex as modern notebooks also depends on how long the software and firmware continues to be useful.
Is coreboot for framework actually being worked on in earnest? My googling only turned up that one guy was doing some basic work on it but only as a concept.
The Chromebook editions use coreboot already, it shouldn't actually be too hard to get it to work on all editions (for Framework). I guess they just have other plans currently.
The Chromebook version comes with coreboot because that's a requirement for all Chromebooks. It would be nice if all editions had that, but IIUC the Chromebook's motherboard is also a bit different.