Responsive design is best practice, except for the most high-end websites
Responsive, mobile-first design gets you the most “bang for the buck” for most web sites or applications. Effectively you can design your web property once, get a good experience anywhere.
But responsive design has its drawbacks, and is arguably compromise solution that works best for small, medium, and probably large (but not super large) web properties.
The Big Boys have different problems
When you’re Google, you see things a little differently. A months-long project to improve the experience for one particular device can seem quite reasonable when you’re dealing with the kind of volume they get.
If money were no object, and your organization could attract as much top talent as it needed, you could start looking at how your web experience could be better for users of particular devices.
“Native” web apps
When most web professionals hear “native solution” we think of a downloadable app designed for a particular platform (e.g., iPhone or Android). While Google has downloadable native search apps, they also go beyond this and also make “native web apps” — website experiences tailored to particular platforms.
Since these specific-purpose web experiences send less markup to the browser than a responsive / adaptive site needs to, they can shave milliseconds off of response times.
For them, few optimizations are premature optimization
Ideally, your web property would not send useless code to the user’s device that just slows down the experience. But that is how responsive sites work. Pages have markup on them for various screen sizes that the user is not using. CSS contains rules for different screen sizes as well. Images that are never seen on mobile devices get downloaded anyway.
Google and other top sites optimize away these inefficiencies by responding to detecting different devices specifically, and responding with just the code they need.
This doesn’t come cheap. They must design, build, and maintain each different version of the site. The continual release of new devices complicates the effort. Even the detection of the various devices is finicky — it’s much less tricky just to build it the responsive / adaptive way.
Google uses browser detection; you probably should not
If you want to see google.com adapt for a different screen size, you need to go into your browser tools and spoof a different device (which makes the browser send a different user agent to the web server). Your website could work the same way, but you probably don’t want it to.
Browser detection to customize code for different devices is discouraged by many experts these days, largely because it is so problematic to maintain. (I don’t particularly want my weekends disturbed because somebody on an XYZ tablet or phone cannot fill out a web form, how about you or your developers?)
Google has shifts of teams of people for this sort of thing, working 24x7. They don’t need to compromise.