Welcome to 2018, friend, where we have several orders of magnitude more computing power and somehow everything is less responsive. It's all accomplished through an advanced process called "Software Engineering".
You can absolutely do a good responsive design in pure HTML+CSS. What seems to happen in practice, though, is pages that appear as flat white if I don't fiddle with my NoScript settings, and that grind on my (admittedly not top-of-the-line) computer if I do.
From a marketing perspective, though, it's not. I saw a startup once where the non-technical cofounders had apparently managed to build a non-dynamic single-page React application as their homepage (basically a basic website but in React ooohhh aaahhh). They were using the fact that they did that as a selling point. Almost all of the non-tech people in the room ate it up completely and praised them for their launch.