You presume architecture firms would have any sort of capability to operate in software development. You might as well say they should create their own ink and papers. Software engineering companies are savvy in software.
Other companies aren't, and it is not expected that they would have any competence beyond their core offering.
Do you have any idea of the AEC industry? Does not seem so.
Funny enough, those big offices signing the open letter are so big they actually have in-house devs. But this is only feasible for the top 0.1% of offices.
Architecture is badly paid and most offices are in the size range of 15–30 people working there. Also CAD programs belong to the most complex software imaginable with really big code bases grown over many years. It's not like you "just write a new one and make it FOSS". You HAVE to rely on others. Also most architects (and even engineers for that matter) are scared by code. They want someone to call and yell at, not looking for workarounds or making a bugfix.