Office 2003 was the last that came on CD, while 2007 and later actually came on DVDs. While not common, 2016 is available on DVDs and presumably the next version 2019 will be as well because not everyone has broadband. You are not even able to customize the installation of 2016 though (well, without editing a file on the DVD itself), so setup full-installs all applications on the DVD.
Removing the ability to customize has been a common theme with Office--2003 was the last that had fully customizable menus and I am still most productive with it. I thought 2007's ribbons were terrible at the time but it probably wouldn't have been a problem if we had today's high resolution monitors back then. The strange thing is 2010-2013-2016 are all much like 2007 with more text labels in the ribbon, so there is next to no learning curve updating from 2007, and thus no pressing need for extensive retraining unless one of the 46% still on 2003.
So it's mostly cost--it does what they want and newer offers no compelling new features. Office is like those Madden games where the only thing they update are the rosters--with Office, the main thing that seems to change is the GUI theme to match the latest OS. And of course defaulting to cloud everything now is truly annoying. So businesses won't upgrade until costs from security issues start to outweigh the cost of upgrading.