There’s multiple reasons. Among the Audacity QA folk, multi-project support has a reputation of a giant can of worms. One clear example is that on Windows and Linux, Audacity allows you to bypass software mixing when accessing the audio capture device (in Windows, via WASAPI exclusive mode, and on Linux, by directly selecting the ALSA device). If multiple instances of Audacity were allowed, then starting play/record in one instance could totally mess up an ongoing recording in another instance. To prevent that from happening, only one Audacity project is allowed to record at a time.
If you are a C++ programmer, it shouldn’t be too difficult to disable Audacity’s lock file so that multiple instances can run at the same time. A little bit of experimentation should be all that is needed to reveal multiple problems.