Developing AI to achieve something particular (here: a speech assistant) is hard.
Google has succeeded also because of buying DeepMind, which did some important breakthrough (not just a Go game engine but also chemistry and other things), and presumably by abusing data of Google search telemetry and libraries used regardless of copyrights. Apple has not bought DeepMind or a similarly successful AI company (this might be Apple's strategic mistake but so can be said about many companies because only a few successful AI companies could have been bought; Apple could have bought DeepMind before Google did) and presumably has not as excessively abused telemetry data or copyrighted material (this is Apple's good, or - considering their iCloud terms - more precisely: not equally bad, behaviour). We should not scold Apple to do the right thing (euphemism) but may for once praise it to attempt developing an AI in a rather honourable manner (except for its PR lies when not being ready yet). Nvidia has a different kind of AI success related to hardware and efficient libraries for it, and has succeeded also because of pursing the AI path for three decades now. Apple was late to AI and this was another central strategic mistake.
Since developing AI is hard, not every company (in particular: Apple) succeeds equally fast. Apple's money does not buy guaranteed success in AI research and cannot compensate having started too late. Therefore, Apple must further delay a useful AI or buy one from another company, as it now does from Google, as a temporary solution until possibly Apple's own AI might catch up some time.
I do not scold Apple for not being at the forefront of AI research because only a few researchers can be there. Instead, I scold Apple for its iCloud terms, by which Apple is by far not as honourable as it pretends to be when not following the top companies (Google, Meta, X, some LLM companies and to a smaller extent Microsoft) of telemetry abuse and copyright violation (AFAIK not Microsoft).
Apple wants to make Siri more relevant and knows it cannot achieve that on its own. As a result, the company will work with Google to rebuild Siri using Gemini technology.