It might not be a problem for DOS 4, but often the source code of software that was only ever meant to be published as closed source contains source code that was licensed from 3rd parties. This license may not allow publishing the source code.

Doing an investigation of what licensed software was used and possibly trying to get permission from the relevant rights holders (if you can even figure out who owns the rights so many years later) can be a big and expensive task, unfortunately. I understand why companies might not want to take that on (even though it sucks).