Frontend applications get more elaborate and smart every day. Part of it is that expectations are higher and higher as well. It is expected for a lot of products that when they launch, they handle complex application state, keep data on refresh, sync everything, give immediate and relevant user feedback, etc. And of course devs don't want to use yesterday's language so complex build tools that transpile newest versions of JS are a given.

Sure, you could build something with jQuery and a PHP backend, and a lot of people manage to pull this off. But you would either re-invent the wheel for some of those features or you'd end up using jQuery plugins to do it for you, which basically is the same as using a module.