Google isn't doing that out of the goodness of their heart though, having a robust app ecosystem is essential and they're doing it for their own benefit. I think there's a strong case that it's a big part of the reason iOS and Android ended up winning against the fairly large number of competing phone operating systems.

So while it's true that there is a financial cost to Google for hosting your app, there's also a financial gain in the sense that the presence of your app contributes to the popularity/viability of their platform.