There is a huge amount of this information maintained by land management agencies (see e.g. [0]). Presumably it is even licensed (in the US at least) in a way that commercial companies could use. But Google Maps coverage of trails and even forest roads is terrible. I presume that you're right, the return is too low, but it is sad that no one is picking up this low hanging fruit. Even OpenStreetMap is hit and miss on this public data (partly because of data import wars).

[0] https://data.fs.usda.gov/geodata/edw/datasets.php?dsetCatego...