I cannot say for sure but I would guess it is also what spurred the ideas to bring in a guy they knew who was doing related work at Stanford and also to acquire what became Google Earth. I believe it all began working with public data. They have obviously added lots of proprietary data since that time. They have a massive amount of cash to spend. Far more than the libraries and government agencies in the US who have the public data.
It all started with public data. Google was born out of a project at Stanford centered around the idea of "online libraries". They had lots of practice using public datasets. The crawlable web is itself a public dataset.
Your point about OSM is the reason I mentioned the generosity of MapQuest.