I think a big thing about AI with Apple is the fact that their stance on privacy makes it a bit harder to compete with the harvested data sets of Google. From what I understand though, according to the reactions on the papers they released a bit back, they're not doing so poorly.
That being said, and I've never used Cortana or Alexa, but I hear they're pretty decent compared to Siri.
Exactly, I think they figure that if they have to harvest data themselves and try to beat Google at its own game, they have a larger chance of losing than if they concede on quality by not harvesting data (as much), but try to offset that by being privacy-conscious. And yeah, perhaps they'll still be able to do a pretty decent job that might not give them too much of a disadvantage compared to Google. You often see, I think, that with a few years delay many machine learning advancements can be reproduced offline.
That being said, and I've never used Cortana or Alexa, but I hear they're pretty decent compared to Siri.