Some people have clung to Windows 7 the way Leonardo DiCaprio should've held on to that door in Titanic. Call it a work-related necessity, devotion to a legacy operating system, or just plain stubbornness, but a fair number of Windows 7 users have simply refused to update to Windows 10. They'll only be able to do that for another year, though, because that's when Microsoft will do its best Kate Winslet impression by leaving all those people behind.
That's right: Microsoft will officially stop releasing any updates for Windows 7 on January 14, 2020. (Technically it stopped updating the base version of the operating system in 2013; users have to install Windows 7 Service Pack 1 to get more recent updates.) Once those releases stop, Windows 7 will have officially been abandoned by its creator, and its users will either have to use the neglected operating system or finally upgrade to Windows 10.
Microsoft simply can't support legacy operating systems like Windows 7 in perpetuity. The company has not only moved on to offering a new operating system, leaving it little financial motivation to update a version it doesn't even sell anymore, but it's also recognized that releasing these updates requires a significant investment. Keeping up with the latest security vulnerabilities, hardware cycle, and driver updates isn't exactly an easy feat.
Hence the current limits Microsoft puts on the life cycle of legacy operating systems. Mainstream support for Windows 7 ended in 2015, six years after the operating system debuted, and the extended support ran for an extra five years after that. Windows 8.1 got the same treatment: mainstream support ended in 2018 and extended support is scheduled to end on January 10, 2023.
None of which changes the fact that some Windows 7 users can't or don't want to stop using the operating system. Many organizations are slow to update their machines for a variety of reasons, from requiring compatibility with certain utilities to financial concerns, so they need their operating systems to be supported for a long time. Some people don't like Windows 10 for one reason or another so they stick with older versions of Windows.
But it's not like Windows 7 will immediately stop working on January 14, 2020. The operating system will continue to function. It will just become increasingly vulnerable to security threats, and eventually it will be even more frustrating to get it working on modern hardware. People who want to keep on clinging to that door can do so until they (metaphorically) drown like DiCaprio. They just shouldn't expect Celine Dion to sing for them.