Because there is nothing that exists today that is completely, from head-to-tail, open source. Being allowed and able to install closed source software does not make an open ecosystem suddenly closed.
Plenty of Linux systems today rely on binary blobs to make hardware work. Plenty of software can run on an open source ecosystem while itself being closed source.
Richard Stallman is a toe-booger eating weirdo looking for attention.
How about: Signal is better? Though, they recently were caught with some unencrypted shit on the desktop client.