Google is actually probably the worst example here (if the most visible): they have the least incentive to share and likely never will. But if OSM becomes the de-facto standard database for mapping (as it almost certainly would if it were a more commercial-friendly license) the mere fact that so many more people are engaging with it on a daily basis would increase the contribution rate.

From a pure logistical sense, very few companies have the resources that Google does. I'm saying it's better for OSM to allow a situation where Google gets something from OSM and OSM gets nothing back from Google because OSM likely would get a lot back from a lot of other parties. It simply doesn't make sense for them to maintain their own data sets in the long term; but in the short term they can't even use OSM so they have no alternative.