It's not. You will detect that in many cases it is a language issue, too. If you look at Lisp, you can see that the language definitions are optimized for interactive use and have a lot of facilities defined at the language level: incremental definitions, evaluation, effects of loading code, a robust interactive error system, ...

How come it is not?

- Create an interpreter library for language X

- Offer a GUI/CLI application using the said library

Not all repls need to work at function level like LISP does.

So you use existing bugs in one given implementation to defend your point of view?

So you redefine missing features due to language problems as 'bug' to defend your point of view?