Comment by nine_k

Prolog is a contrived example. It's small and simple enough to be implemented as a DSL embedded in another language, reusing most of the parser and leaningn its execution logic.

Now try to produce a library that adds compile-time features: static types, lifetimes, the notion of const and constexpr, etc. You can, of course, write external tools like mypy, or use some limited mechanism like Java annotations. But you have a really hard time implementing that in an ergonomic way (unless your language is its own metalanguage, like Lisp or Forth, and even then).

Creating a library that alters the way the runtime works, e.g. adding async, is not entirely impossible, but usually involves some surgery (see Python Twisted, or various C async libs) that results in a number of surprising footguns to avoid.

Frankly, even adding something by altering a language, but not reworking it enough to make the new feature cohesive, results in footguns that the source language did not have. See C#'s LINQ and exceptions.