Because they're not compatible with new hardware and very much more limited in term of features compared to more recent stuff. Can't even install win 7 on a new computer these days.
You can still use them with emulators though. I have super old ms-dos games I still play today.
Stuff gets thrown out or replaced once it outlived it's usefulness. Companies do like to hold on to super old software though and they either use old hardware to run them(which is an issue once it breaks) or they update the old software to run on new hardware.
I've had to leave an old computer behind a few times when doing migrations to 64 bit win 7 back in those days cause it stopped supporting 16 bit emulation and those companies still had a 16 bit software they didn't replace.
I also know a banking company that still use the same software they did back in the OS/2 days(1987), it was simply updated a few times to run on new hardware.