For instance I wanted to be able to display PDFs directly in VSCode. I went to look for a plugin, and there was one. People simply used "pdf.js" to integrate PDF support in VSCode. Because it is web based it should have been straightforward to do. Doing the same with native technology would have took several weeks of coding, and it wouldn't have been cross-platform.
Imagine all the web-based open-source tools that could potentially be integrated in these editors. Integrating a SVG editor in native text editors would be a nightmare. With web-based text editor you can potentially just incorporate an existing tool like [1].
There are lot of other examples like this: Live markdown preview, mini-map, color-picker, integrated VSCode debug panel, …
It is also quite easy to add visual stuff. For instance adding a vertical bar at 80 characters in the background is quite easy to do with web technologies. On the other side emacs has still not managed yet to make the html-mode work nicely with this "fill-column-indicator". It is also probable that it would be much more easy to integrate web services (trello, github, …) directly within VSCode.
At the end people wanting performances have already quite a lot of choice in native text editors (vim, emacs, sublime…), and people who prefer functionalities can go with web-based text editors (atom, vscode, …)
[1] https://svg-edit.github.io/svgedit/releases/svg-edit-2.8.1/s...