Why? Google is a low-quality road map for cars. They don't put much effort into walking. In the OSM community Google is considering a very low bar. We aim to meet or exceed the quality of mapping like Ordnance Survey.
A goal of OSM is to map _what we want mapped_, not what makes commercial sense to map.
I'm not saying OSM is more complete than Google maps, just that the incentives are different and therefore the coverage is different.