To your point, here are two adjacent "parks" in Washington DC. The park on the left is fully open and walkable. The park on the right has a low fence and shrubs surrounding it. You could technically hop the low fence and walk there, lots of people let their dog do that so it can crap there, but both practices are discouraged.

Also, if you are mobility impaired, or its muddy, or whatever, you wouldn't want to walk in the park on the left either. So it makes sense that OSM wouldn't try to shortcut you over it.