Vanilla html/css/js are great for static sites, but once you start building an application it gets more difficult to manage state: both on the page, and with th

Vanilla html/css/js are great for static sites, but once you start building an application it gets more difficult to manage state: both on the page, and with the backend that stores user data. What you're saying is similar to saying "I don't understand why someone would use Django, can't I just use the python http.server for an API and psycopg to connect to postgres?" The answer is yes, you can do that, but over time you'll find that there are pitfalls with a bare-bones approach that are easily avoided by using higher-level frameworks. Mostly the spaghetti that someone else mentioned, but also getting caught up in technical details that the frameworks have mostly already abstracted away.