A lot of microsoft's cloud expertise comes from running bing. Also how do you expect them to push stuff like cortana if bing (the brain) shuts down.
It aligns directly with each's desire to deliver products and services (yes, even Apple - music suggestions is one example) rather than just be a hardware/software retailer.
The virtual assistant's job is to pick a supplier and spend your money on your behalf. Everybody wants a piece of that.
They won't get users to say "Siri, book me a flight" if it books you a $500 American Airlines flight when you wanted a $300 Southwest flight.