Why don't Google and Amazon take over online travel?

They're big and they buy things. Why aren't the tech giants taking over online travel? Or maybe ... they already have!? We also talk about the illusion of choice when it comes to booking online. Major players like Expedia gobbled up other travel sites, so when you're shopping around, often you're just browsing different floors of the same department store. Plus, it's only a matter of time before Airbnb starts listing hotel rooms. What's stopped them so far?