When OSM got started, the idea was to use GPS track data almost exclusively, or at least for the finer resolution stuff. The satellite imagery available freely

When OSM got started, the idea was to use GPS track data almost exclusively, or at least for the finer resolution stuff. The satellite imagery available freely at the time wasn't that good anyway. But cleaning a bunch of GPS tracks can be a real mess. Many GPS tracks don't include the error/accuracy measurements and just pretend to be infinitely-small points scattered in the general vicinity of what might be a road. I think people tend to overestimate how accurate their satnav or phone's GPS is, because in most uses it's "locked" to a road or other map feature. Anyway, not an intractable problem by any means, but preprocessing and cleaning GPS data to derive the actual terrain can be quite difficult. Paying somebody to click repeatedly on a satellite image might be temptingly easier.