In order for an operating system to be endorsed by the Free Software Foundation, it must meet the GNU Free System Distribution Guidelines. The issue is that these guidelines are flawed and idealistic, and don't represent the best way to encourage adoption of the principles of free software. Entirely free distributions such as Debian are not endorsed by the FSF because they do not meet these guidelines.
Richard Stallman, and by extension the GNU Project and the Free Software Foundation, see the issue of free software as a binary issue: you're either all in on refusing to subject yourself to proprietary software, or you're not. Of course, this binary approach works when determining whether a program is free software (it either is or isn't), but it doesn't work in the scenario of user adoption of free software.
I'll speak from experience to prove my point. My first time dipping my toes into the free software world was back in 2016, when I installed Ubuntu onto my computer and educated myself in the GNU/Linux world. I didn't do this because I wanted to ditch proprietary malware, infact at the time I had never even heard of the free software movement. I installed Ubuntu because I knew that GNU/Linux was more customizable and tweakable for a power user like myself, and I loved hacking my system to fit my needs perfectly in every way I could. This later threw me down the rabbit hole of GNU/Linux, open source software, and by extension free software, and even my ideals on consumer privacy. The point is that this may never have happened if I knew that by installing Ubuntu, I'd be trapped into only using certain programs that were deemed "free", and I would have to relearn everything.
This is where the flaws of GNU's Free System Distribution Guidelines become apparent. In order for a system to comply with these guidelines, its channels must not allow for easy installation of any nonfree software. To distribute nonfree software that may be popular, or to even provide instructions on how to install nonfree software or firmware, is a violation of these guidelines.
This shows the binary thought process that the project has. Instead of encouraging any and all adoption of free software, they imply that if you're not 100% committed to the idea that any and all software that you use must be completely free right now, then you might as well not use free software at all.
Many users use Spotify to listen to their music; use Microsoft Office for document editing (or may even be required to use Office because of their job); use Photoshop for image manipluation. Any and all steps towards reclaiming freedom should be celebrated. Their efforts aren't worthless because of a couple of programs that they may still use.
The Linux distribution Debian, although shipping with absolutely no proprietary software or firmware, contains a "nonfree" package repository from which nonfree applications can be installed. This repository is disabled by default, and must be enabled by the user willingly before it can be used. However, the FSF sees this as non-compliant with the guidelines, and as such Debian is not endorsed by the FSF, despite their efforts to give users the freedom they are entitled.
Regaining freedom in the computer world doesn't happen in one night; it is a slow and sometimes painful process, and that's okay. If we want non-tech-savvy users to become a part of the free software movement, we need to let them take baby steps, and acknowledge that they may continue using some nonfree programs for a very long time.