I'm not a web developer, but I dabble in it because I find it interesting. I went through some courses like The Odin Project and others to learn the basics of HTML/CSS/JS, and I thought it made a lot of sense: HTML structures the page, CSS handles the style, and JS handles the functionality.
However, after working for software companies for a few years, it seems like almost nobody uses these technologies in the way they're presented on websites like w3schools to make web applications. Instead, they use component-based frameworks like React. To me, these frameworks seem way more complicated than HTML/CSS/JS, and I don't understand what problem they're trying to solve. What makes the trade-off of all that extra complexity and abstraction worth it? Aren't HTML/CSS/JS perfectly fine and time-tested tools for web development on their own?
That's it - that's the problem they solve. People started writing vanilla HTML/CSS/JS and struggling to make "reactivity" (hence React) work well when web applications got complex with a lot of state to manage. So we started building frameworks on frameworks and abstractions on abstractions and ended up where we are today.
It's gone a bit far but the overall purpose is a valid one.
(From someone who has used React professionally but uses vanilla HTML/CSS/JS for his personal website)