I think I would rephrase this question as why would I stop using emacs for VS code and intellij? Seriously, there is no feature they have that my emacs config doesn't (AFAICT after using Intellij and VS code each for 4+ years). But I have the ability to work with emacs without ever touching my mouse. You may consider that only being "slightly" faster, but every time I use VS code, _feel_ slow because I know how fast I would have been in emacs.

Combine this with my tiling window manager and I pretty much only have to use my mouse to interact with web browsers. That's the killer feature that makes VS Code and Intellij _feel_ inferior to me.

Now I wouldn't go so far as to say that VSCode and Intellij are _actually_ inferior to Emacs. I think both are great in their own right, but after years of familiarity I have a fluency with Emacs (because of the keyboard oriented interface) that I could never have with them.

To be fair, for someone without emacs fluency I would probably steer them towards VSCode and Intellij as they are definitely more approachable. In fact, I have done exactly that for several of my friends and colleagues.