Why can't Synth ascension assimilate Cyborgs? Makes no sense.

I mean, my species clearly has the knowledge on how to advance from cyborgs to synths since that's what you do normally, so why can't you do it to other cyborgs? This puts you in a weird situation where completely organic pops are better to conquer because you can bring them into your glorious synthetic evolution, but pops that are already halfway are stuck and sit around complaining about food. Stupid Materialist FE pops, tempted to just console them out on principle.