Gatekeeping - nobody else can be the default voice assistant or power Siri, so where does this leave eg OpenAI? The reason this is important is their DOJ antitrust case, about to start trial, has made this kind of conduct a cornerstone of their allegations that Apple is a monopoly.

It also lends credence to the DOJ's allegation that Apple is insulated from competition - the result of failing to produce their own winning AI service is an exclusive deal to use Google while all competing services are disadvantaged, which is probably not the outcome a healthy and competitive playing field would produce.

Apple and Google have a duopoly on Mobile OS. If Apple uses Google's model for Siri, that means Apple and Google are using their duopoly in one market (mobile OS) to enforce a monopoly for Google in another (model for mobile personal assistant AI).