Why the Web Server do not sends all the resources at once?

From the reference at MDN, I was watching this very first video on Web Demystified : https://www.youtube.com/watch?v=O_GWbkXIqEY&list=PLo3w8EB99pqLEopnunz-dOOBJ8t-Wgt2g&index=1

I understand the simple process that when a resource is requested via a URL typed on the web browser the request goes to the appropriate web server which then returns the HTML and then browser requests further more resources to the web server like JS, CSS and Image files that it got to know from the HTML it received.

I was wondering when the server already knows that this has to happen, why shouldn't it send them all in one go. Anyways the browser is gonna request them next.

Won't this make the job of server easy ?