but how come the linux and BSD kernels, emacs (since RMS), the java language, even python (python3 was not a rewrite), git, hg, django, etc ... have never been

but how come the linux and BSD kernels, emacs (since RMS), the java language, even python (python3 was not a rewrite), git, hg, django, etc ... have never been rewritten from scratch? what is the lesson here?