"Because the map doesn't belong to you. OpenStreetMap is not a private registry for land managers or trail builders to curate what they think the public should or shouldn’t see. It’s a global, open, factual map of what exists on the ground. The job of OSM is not to enforce your access policies or preserve your status quo—it’s to reflect observable reality, transparently and accurately."
This is not the strong argument you think it is. It is a statement of what OSM does, but ignore the reality of your work. 

" If a trail is visible, verifiable, and walkable, then it belongs on the map" - WHY? No one here can ever give a answer to this question. 
Mapping is not a neutral action. 
You are damaging trails and communities and the work that has gone in over decades to achieve the status quo. 
Mapping a trail or something like a secret sauna then shrugging your shoulder afterwards and saying, "oh well, it is what it is, mark it as removed/closed/etc" is just incredible in it's ignorance. 

"Pretending something doesn’t exist by keeping it off the map doesn’t make it safer, doesn’t reduce usage, and doesn’t protect your community."
It absolutely does all of these things. It's incredibly easy to trace the closure, destruction and increased maintenance load of trails that were off map and are now public. Or get promoted through vapid instagram influencers. 

"It just makes users less informed. And that’s the real danger."" - Downstream consumers of the OSM baselayers do not get the tags, trail ratings, or other meta data associated with the previously off- map, trails. Surely you can see how this is now problematic? If i have no idea this trail exists, I'm not uninformed, Im unaware. Now I see it on the map but have no idea that it's a proline trail and it's easy for me to get in over my head. The latter situation is immensely preferable. 

"That’s how responsible mapping works." - Well in that case OSM is not being a responsible mapper as trails are frequently added without proper naming, difficulty data, warnings, nor ensures that downstream consumers are acting responsibly when they get the data.  If anyone here really cared about responsible mapping then these things would all be considered and they are clearly not. 

"convince the community of your argument."  - Lets be serious now. No one should have to convince the community. Your community doesn't and shouldn't override the wishes of the community which you are having an impact on. To suggest that one or two OSM users rights and wishes overrides that over hundreds of local riders, builders and advocates is absurd and offensive. 