Since season 8, Richard owns a new gadget: Lucy. A small pyramid on the desk at the kitchen, glowing blue and talking with the voice of Aubrey Plaza.
Why do they develop things like this, when they hold a iPhone with Siri on it in their hands?
Licensing a known property such as Apple's Siri is a time-consuming and potentially expensive proposition for any show. The fact that you then want it to exclusively do things that Siri can't do (operating and running a smart household, acting as a counsellor for Castle, interrupting would-be murderers) makes that doubly difficult.
Siri is basically a glorified search app, not a home OS and lacks any of the sophistication shown by Lucy. On top of that, having Siri as his home manager means that you don't get to cast a popular actress like Audrey Plaza into the role.