Because Apple doesn't care. Siri is just something that is bolted on when you buy the hardware. Cortana was an actual product that Microsoft is trying to sell.

I don't know anybody who uses Siri for more than just setting an occasional timer or something. Usually it's a nuisance when she is accidentally activated and takes over the interface.

Apple is stubborn.

And to Apple, Siri is to Siri AI as MacBook is to MacBook Pro.

Completely different product.