> simple I/O operation

That’s the reason it doesn’t get implemented. Nobody wants the simple I/O operation based inclusion. The moment you try to propose it, there’ll be demands to add conditional logic or macros. More relevant for the web is instantiating html templates, which will undoubtedly get piled onto such a feature. And pretty soon you have yet another:

> interpreter in your browser

Might as well use the one already there.