It splits the community and number of possible volunteer hours for one. It also splits the canon into different versions. More projects fight for the attention attention (and possibly donations) of the audience.

There are lots of reasons it could be preferable to centralize. OTOH their mission is limited and some competition is healthy, if only to explore alternative ways to do things.

It’s a different mission.

PG focuses on an accurate digital translation of the source material, sometimes hosting multiple different versions of the same text, and doing things like putting work into recreating the adverts at the back of some novels.

SE focuses less of preservation and more on making readers’ versions of the texts, like other publishing imprints. So there’s typography standardisation, a light-touch moderinisation of hyphenation and soundalike spelling, and things like author-wide collections of short fiction and poetry even if it didn’t previously exist.

Both are valuable, but they serve different segments.