If frontend was bicycle science then I must have missed that in my 10+yrs writing frontends with JS+CSS...
> They make it easy to get 80% of what you want, but that last 20% is bear
JQuery was the standard before JS frameworks took off and it was a nightmare for anything besides smallish websites. It was good for 25-50% of what you wanted at most - everything else was completely different every project you looked at. Trying to manage a large application that any serious modern web-based SaaS company uses was incredibly difficult.
I started using Backbone.js before Ember/Angular was even a thing yet and it was the first serious attempt at building large JS-driven apps for browser and mobile. That was simpler because it mimicked Railsy REST/MVC set ups, plenty of analogies for backend people to learn from, but that turned out to be a poor way to create UIs and React/Vue/etc + Redux offered a thinner view layer with a very clean way to manage state and bind data to the UI.
JS frameworks stopped pretending they were traditional web server app/database in the browser. But rather one that reflected the reactive interactions which we are trying to model in real-life spread across various individual components... more like desktop interfaces, not recreating web apps backends in the frontend.
Vanilla javascript has improved quite a bit to lessen the need for JQuery, but even then managing state and mutations reliably, modelling data, coordinating async operations, only loading the JS you need on a particular page, code organization/modules, etc have all been made way easier with the new Webpack-driven frameworks.
But you can still use simple vanilla/JQuery if you only have a small project. There's still plenty of Jquery libraries being developed and simpler UI libraries like Bootstrap...
Unless you mean pure-HTML forms, which are extremely limited in terms of interactivity. But again they haven't gone away either if it fits your needs.