While it isn't pretty, you can drop Vue in a <script> tag on a page and define components with template strings.
In React, you can technically do this, but you need to either:
A. Use React.createElement() calls
B. Use something like "htm" also included via a <script> tag, with template literal tags to wrap your component renders
You can technically include Babel in a <script> tag to use JSX, but it only works with <script> blocks that have been tagged as "JSX", not within regular ".js" files you can import using "src="
Jason Miller a-la Preact is the person I see most consistently working on and pushing this sort of React/Preact no-build setup. I definitely get the appeal, for this exact usecase -- you have a simple, mostly-static website (or something old and built with jQuery) and you want to incrementally modernize it/add functionality without rewriting the whole thing.
You can go pretty far using Vue with a <script> tag before you start needing build tooling.
what you're responding to is marketing, which Vue has done an effective job of. thats fine, but it is unfair of you to conclude "Gradual adoption is a feature / selling point not many web development frameworks can claim."