This question is irrelevant in the context of negotiating a collective bargaining agreement. The WGA's motivations are obvious. They are protecting the livelihoods of their members.

If a studio thinks AI can genuinely do a better job than human writers, that studio has the right to replace those human writers with AI. The human writers likewise have the right to refuse to work with any studio that does so. The fact that no studio has even hinted at taking this route during a 5-month-long strike suggests that no one believes AI would actually do a better job than WGA members.