I understand it might seem confusing if you are not familiar with the requirements, but they are not trivial to bypass.

Cell phones operate in licensed radio spectrum, so they need to have proper testing and certification (https://www.fcc.gov/oet/ea/rfdevice). Any device not properly certified would be illegal to manufacture or import into the US.

Separately cellular networks require PTCRB certification of the devices to ensure they are interoperable with the network (https://www.ptcrb.com/). The FSF could in theory write custom firmware for baseband and wifi chips, but they would need to seek certification as this would be considered a substantial modification. It would likely require cooperation from the chip manufactures to provide samples with various testing/debugging harnesses enabled.

Qualcomm and the like would probably sue to stop the FSF on the basis that it could put their own device certifications into jeopardy.

That isn't even touching on non-transmitting components like GPUs or sensors where the actual functional logic may be split between hardware and software (your blob driver). Even by doing a clean room reimplementation, you risk infringing on software patents, and will have little flexibility to work around them since the hardware will expect things to be done a specific way.

You would think it would be ridiculous to assume the people working on this know nothing about the mobile space, yet their actions do bring that into question.

I think all your concerns are valid but they are not necessarily insurmountable. The FSF or whatever other entity could do just what you suggest and seek certification within the current legal frameworks. They could also talk to the carriers and negotiate individually which is probably going to be quite annoying and slow but it's not impossible and it's not like that's not done in the commercial space. The could build mechanisms into the hard-/firmware that takes your device off whatever regulated spectrum/provider if you modify anything that is in regulated territory (as watched over by some form of maintainer-quorum-signing-negotiation-structure). I'm sure there are many mechanisms and processes one could come up with that could keep with regulatory or other control aspects while still keeping things open.

All that patent and legal business is probably a more important/existential concern and a go/nogo-factor if you want to be a commercial player in a market-driven environment and less so for an entity like the FSF.

This is aounds similar to the objections that were raised against the GNU project in the 80s.