In an interview I read, Tarn said something to the effect of that "he was not interested in learning about version control and it would just be a distraction that would take away time from the important work, since the way he works has worked for him for many years".
I can't help but think that first, it's a bit dangerous way to work, all "past, current and future" state of the project is only in his head and if he happens to become indisposed, the project is very very likely to die (though I suppose there would be hardcore fans willing to reverse engineer it)...
Second, it's a loss of historical record... imagine in fifty years historians wanting to analyze the project's history, it would be so much more useful to have a granular history instead of just a blob of files scattered across thousands of directories...
Just look at Unix History repository here for example: https://github.com/dspinellis/unix-history-repo - I think there's immense long term value in preserving this kind of archival history.