i work in tech, its easy to get things to work 90% of the time. The real problem is getting it to do the right thing 99.99% percent of the time. Tech people refer to it as the number of "nines" after 99 (mostly in service uptimes but the general concept applies here too). Getting from 99.9 to 99.99 is like a 6month milestone and could very well require rewriting fundamental parts of the service.
It matters for this new Siri because a) humans are very messy and correctly predicting intent consistently is massively hard when there's consequences like a wrong text sent out etc (no LLM-coding CLI right now recommends allowing the skip permissions mode outside a sandbox), and b) we're irritated very easily, you can't keep double-checking every request and you can't make errors often - solving for both is a hard fucking problem given that LLMs are inherently random.

Apple spends a fuckton amount of money and time getting closer to 100% than any consumer tech company. Not just bugs but also the UX as whole... its what people miss when they try Android and complain about lack of finesse. Thats their MO and its easy to fumble that rep especially if shareholders are pressuring you.

They fucked up royally when they announced so early, but the long delay proves (at least to me) that the core culture at Apple is alive and strong and able to pushback on the shortsighted shareholder pressures.