JS devs usually need quite a few threads/processes. The editor/IDE is 1 or more processes. Compiling is a separate, multi-threaded process. The webserver is another process. Tests are several more processes (which matters because 30+ minutes on a fast desktop isn't unheard of when running a complete test suite). Parsing the newest compile is multi-threaded in the browser. A separate core for the main website thread is especially important because opening the dev tools de-optimizes all the things resulting in significantly slower performance (a big thing with web apps). If service workers or webworkers are used, then there are even more processes involved.
Going from my dual-core Pixelbook to my Ryzen desktop is a huge change in the development experience.
It could be argued that some "developers" should be restricted to pencil and paper, but it's a different issue. Even hipsters deserve to multitask at their computer.
