The problem is a business/orgnanizational problem - it's not like developers are incapable of mimicking their own websites with pure html+css+js (especially with all the improvements to JS and the browser apis over the years), it's that doing that and then maintaining and growing it with a team of people is basically impossible at any useful scale.
Frameworks provide a common language for teams to build their site with. The fact that they also currently introduce a lot of extra complexity that require developers to handle previously "free" things (ex. performance, bundle splitting) isn't some fundamental flaw with the idea of frameworks, it's just a result of people still trying to figure out exactly how the hell you can provide a nice, interactive site with all the bells and whistles that'll make like 5 different groups of stake holders happy.
If "React" is used loosely here to mean a modern frontend framework, then at least one concrete benefit is the ease of splitting web pages into components, and the ease of co-locating CSS with these components.
I have tried Eleventy with nunjucks, but compared to the way React (Vue, Svelte, whatever) allow you to organize your code, it felt really awkward. Even Google dev rels, such as Jake Archibald, admit that they love the developer experience of something like Next.js [0]
[0] https://developers.google.com/web/shows/http203/podcast/soci...