That is not an argument. The same implementation could be used to highlight SQL queries in arbitrary code, for example. What you want is the general ability to have regions of nested code in a file that use a different language than the rest of the file.
VSCode does support that. It's up to language extension authors to implement it though because obviously only they know which bit of text is in which language.
I don't think SQL queries in code is a very big use case and even there, any that are large enough to need syntax highlighting are probably better off in a separate file.
The biggest use case of multi-language files I've seen is Vue single file components, but in practice they turned out to be a terrible idea precisely because of tooling issues like this.