Because I gain nothing when writing in the editor window - because in _Emacs_ the REPL window works (almost) the same as the editor.

A REPL with Paredit, autocompletion and 'normal' editing capabilities is what I want. That's why I often program other languages (that have a REPL, but no way to comfortably edit using it, like Haskell, PureScript, Elixir, OCaml und Rust) using Jupyter notebooks, which is the same as evaluating inline - that's better than the 'normal' way (only seeing errors/warnings/... inline) but it's not the same experience.