And it's because there is no standard for an adaptive interface. The only thing standard, really is an ever-changing menu bar which Apple did first, and Ubuntu took wholesale for Unity.
Not saying that Apple makes standards, but it's pretty compelling to say Apple influences what companies do, despite having such a small market share in comparison. Consider this idea; Apple had a huge hit with their MP3 player series known as the iPod. There were so many people with iPods that it was outnumbering other MP3 players in some regions. Microsoft later tried to compete with the Zine and despite it being potentially superior it flopped.
Apple used the momentum from the iPod to launch the iPhone. Lots of people were on clamshell phones but despite other companies trying to do what Apple did first, Apple made smartphones a "Popular" thing to have. iPhones would later displace PDAs which were probably inspired by the Apple Newton.
Even the PC market has Apple written all over it, which is why laptops nowdays are coming with fewer ports, and are becoming thinner than ever instead of just being the same. I personally hate it, but that's because I want a laptop that can survive being in a backpack without having a hardshell case to go around it.
Despite Lenovo being an absolute flop with its adaptive interface, I've seen HP try to do something "Similar" (albeit a faux approach) with an Pavilion dv4 which has a capacitive touch surface similar in form to the Apple touchbar. If Apple's laptop is popular enough, I anticipate there will be a PC standard which Linux could lead the way in heading for Microsoft to later implement in Windows, but developers need to code the support in, and there needs to be an open-source standard for this kind of functionality which other companies can take and implement as they wish.
I personally like the idea of developers coding for a universal standard that can simply reside in a toolbar and could be enabled by the user on a per-application basis because not all applications will need such functionality, nor the user would want such functionality for all applications. And it would provide a use for the empty space in most GTK dual-panel interfaces.