Quite frankly I think that is the primary reason why Apple hasn't care much about Siri. It's not a make or break feature, very few has probably switched to Android because Siri sucks. Not improving on Siri has cost Apple very little.

Right now shareholders expect a visible AI play. Apple doesn't have much in terms of visible AI, there's plenty of AI features in e.g. the Photo app and Mail, but investors want a chatbot. That's what the market thinks AI looks like and Siri is what Apple has that fits the bill.