They can't distribute firmware blobs simply because FSF and GNU do not in principle participate in distribution of any non-free programs.
Also consider that if a manufacturer can distribute opaque firmware updates to your system, it practically has remote control over it, е.g. Intel can activate a backdoor in specific CPUs when needed by publishing a microcode update.
Also consider that if a manufacturer can distribute opaque firmware updates to your system, it practically has remote control over it, е.g. Intel can activate a backdoor in specific CPUs when needed by publishing a microcode update.