I wonder why do those people even care.

The only time I look at my resource usage is when apps start to behave funny. Or when it's an app that I am developing. Other than that, why should one care?

One argument could be made that they are using memory wastefully. Not sure that's the case. The baseline memory consumption is higher, so what? It's a tradeoff. It's easier to build a better editor using browser-based tools, as much as I like Elisp. Over time Atom and VSCode will close the gap.

Now, if there is a memory leak, or memory increases non-linearly with the workload, than it could be a problem. VI and Emacs are pretty great with large files (Emacs not so great with long lines), browser-based editors usually do not work as well. But there is no reason they shouldn't, it just takes engineering effort.