Why do people still ride horses? We don't need them, yet some people own and care for horses. Some people still drive Model T Fords, and other old things.
I still want a Gray Planer (a huge old obsolete machine tool) taking up a huge chunk of my garage, for zero practical reason. Though I'd settle for a benchtop shaper.
A lot of programs were written back in the days of MS-DOS that people got used to using, and they still work. You can't directly run MS-DOS programs anymore. You used to be able to right up to Windows 7, 32bit.
If you've got an old program that still does a job, you're used to it, why change anything?
There are programs that talk to hardware that run on MS-DOS, those programs and hardware represent a huge investment of time and effort, writing the program, building the hardware, and especially debugging the system. If you have those systems in production, you're going to do every single thing you can just to keep them running. You are certainly not going to risk any changes to a working system in the name of efficiency, especially if your business, or job, depends on it.
In many, many cases the original people, documentation, and decisions that went into the design are all unavailable, and all you have left is the artifact that works reliably if you maintain it and feed it proper inputs.
Museums and Archivists need to run old software to be able to retrieve old documents. For most cases, you could use DOSbox, or QEMU , but sometimes you need the old programs on the old hardware.
Finally, and likely most importantly, is nostalgia. For many of us our first real computer was an IBM compatible running MS-DOS. Because you can't buy DOS any more, FreeDOS does the job.