For me, it's because they add so much complexity for what I see as a minimal gain. Frameworks like React and Angular allow you to create reusable components in your web project, but at the same time, completely take over your front-end. This might not necessarily be a bad thing, it could be a good thing if that's the way you like to develop, but for me, I don't like it.
I'm not opposed to the idea of reusable components, I just think they should fit in to your project, not define it. This is where framework-agnostic web components come in. They can be used with any framework, or none at all.
In my opinion, that's the way it should be. I use a tool like Lit-Element to make web components that work regardless of what other tools I decide to use or not use.
That seems like a "dream situation" to me. I can still leverage web components while not having to use a full-featured framework if I don't want to. And if you feel differently about it, write your thoughts in the comments if you feel inclined, I'd like to see what you have to say!