They are seperate because they both have different motivation, tactics, and origins.
Open source could also be used to describe a development methodology (public repo that accepts pull requests/patches with a license that allows redistribution). Free means that the user is entitled to all 4 freedoms (use, study, modify and distribute, or redistribute).
The Free software movement works to create a world of entirely free software. Open source initiative does not make that claim. OSI is more pragmatics (at a cost) while the FSF is more ideologically focused (likewise)
We have this distinction because it matters and that it reduces confusion. GNU doesn't go "shove it."
moments there.