In compileall.py's main, we verify that the provided destdir (-d) exists at build time.  But destdir will commonly be used to override the build time path with a runtime path.  That runtime path will usually not exist at build time.

Note that this logic was changed when compileall.py was migrated to argparse.  I think the old logic accidentally avoided the isdir() check at build time.
https://github.com/python/cpython/commit/11e99b06bda2a23478fcec40df8c18edc8a06668

With the attached patch, behavior is made consistent with python 2.7, intended or otherwise.