I've been using Calibre on all my devices for years and have hundreds of books annotated.

Why would I switch to Foliate?

Not OP, I haven't used Foliate either.

Calibre UI is unappealing and too complex. Sometimes you just need a library of books without the clutter. Foliate looks simpler to use, but for some reason it doesn't allow bulk imports.

There is calibre-web[1] which some might prefer.

[1] https://github.com/janeczku/calibre-web