Good question, they considered it at some point:

> Lilypond is a very good tool, but the part on Gregorian chant is not maintained and very deep modifications would be needed to perfectly align the notes and text.

Source: https://gregorio-project.github.io/gregoriotex/index.html

Circa 2024[1]:

> Note, however, that there are some serious flaws in LilyPond regarding Gregorian notation (especially the non-modern version), and right now there is nobody who works on improving that...

1: https://lists.gnu.org/archive/html/lilypond-user/2024-10/msg...