That may seem to be true some places, but I've seen many places (including around my house) where Google has great coverage of nameless walking and biking trails that aren't open to cars.
Consider for instance this city park in Seattle:
https://www.google.com/maps/@47.6397466,-122.2948284,18.95z
https://www.openstreetmap.org/#map=19/47.63987/-122.29488&la...
Google has gotten better in recent years, it now has a few of the walking trails, but compared to OSM it's still absolutely pitiful. Even in high profile locations like Yellowstone National Park, Google Maps is pitiful compared to OSM:
https://www.google.com/maps/@44.3925569,-110.5541906,18z
https://www.openstreetmap.org/#map=18/44.39273/-110.55472&la...
Or what do you see that the rest of us don't? :-)
Given what Google uses to build its maps, that's not surprising:
https://d.newsweek.com/en/full/259216/google-street-view.jpg
Both fall well short of OSM for topics other than finding local businesses. OSM gives me incredible detail, even telling me where water fountains and benches are in the local park. Meanwhile Google Maps doesn't even recognize the restricted access road running through the park, let alone any of the walking trails, let alone any water fountains, benches, etc.
If you can't drive a car on it or through it or to it, Google thinks it may as well not exist.