In my spare time I have been writing a library to render standard macOS controls with Liquid Glass.

It's shown me why they use it for the backgrounds: it's so heavily based on refraction (oddly, mostly on one axis, not sure if this is obvious visually!) that for something like a button, there's nothing to refract, therefore it looks very plain. You have to have a background for it to look good.[*] I can't help wondering at what happened internally: my personal pet theory, for which I have no evidence, is that someone missed Aqua, thought 'if we do it with shaders it will be new and shiny and we can release it', and that tech decision forced implementation aka glass backgrounds not foregrounds.

[*] My solution: add a background to buttons and other controls. I'm going for a look inspired by Lion. It doesn't have to be very prominent; it just has to be there for something to refract so your eyes see shape and recognise the glass.