Sometimes it's arbitrary though: You have places like Ben Lomond where your route relation goes up the main tourist path and then down the Ptarmigan ridge, but that is no more or less correct than going down the same way you came up (which as far as I know is what most people do), and there are certainly no signposts on the mountain telling people that to complete the "Ben Lomond Trail", they need to go down the Ptarmigan Ridge.
https://www.openstreetmap.org/relation/16124656

I think what we are missing is an open data equivalent of Alltrails, where you can just upload any trail you want.

Re "goat trails": you can improve the map a lot by adding sac_scale and trail_visibility (for paths that you are familiar with). This is probably more valuable than adding route relations because it gets shown in a lot of hiking apps. But it relies on someone walking the "goat trail" and then adding those tags based on their experience.