Android first released software designed specifically for tablets in 2011 with the introduction of Android 3.0 Honeycomb. It featured a whole new look and some elements were designed to work better in larger screens. Not many tablets adopted that OS version, but it served as an inspiration to the phone platform which would later be known as Android 4.0 Ice Cream Sandwich. In 2012, Google partnered with Asus to build the Nexus 7 tablet. It ran on the brand new Android 4.1 Jelly Bean, which instead of being designed for that tablet, it was meant to unify the interface between tablets and phones. Even though the Nexus 7 was a smaller device meant to be used in portrait mode, this new approach was evidenced in the Nexus 10, which was introduced a little later and was meant for landscape use, so the whole UI was optimized for that mode.
A long time has passed and there are a lot of tablets running recent versions of Android. The most recent version is Android 6.0 Marshmallow, but the approach is still the same, the operating system scales to screens of all sizes. Still, even when some apps have been optimized for the larger canvas, making better use of the extra space, the vast majority of apps look like blown-up versions of the phone apps. Google's Play Store even included a special section for the tablet-optimized apps so developers felt encouraged to redesign their own apps for these larger screen devices.
With the recent launch of the Pixel C, a 10-inch tablet designed entirely by Google and supported directly by them, Glen Murphy from the Pixel C team has answered a bunch of questions from some users on Reddit. One user asked him about their strategy towards encouraging developers to make their apps compatible with tablets. Murphy answered: "We're spending a lot of time working with developers to get better and more awesome tablet apps, but it is definitely a chicken-egg problem, and we think a key driver is awesome hardware (like the Pixel-C :)". While the Pixel C might be the most powerful tablet right now, it seems a little late to take such a measure because the tablet market has been declining (that's not to say that there aren't many Android tablets out there). But perhaps now that Google is in charge of a tablet, they might take additional approaches to make this happen sooner rather than later. Ultimately, users would get better-optimized or redesigned apps for their Android tablets.