@rejeep I am not sure.  We could surely provide an option for this, but don't think that we should clutter the package directory with auto-generated files by default.  After all, the package directory is likely to be kept under version control, and who wants auto-generated files in his VCS?

Besides, the index needs to be re-generated as the contents of the package directory change.  If we auto-generate an index file, we would need to make sure that it is up to date, i.e. by tracking the modification dates of all packages and comparing them with the auto-generated file.  That's a lot of complexity for little gain.

I think, we should better leave the index file generation to the user.  In a big archive (i.e. MELPA), the index must be pre-generated anyway, because even the first index generation is likely to be too slow.