First: Your edit should had rather been a reply. Also it should had contained where you found what you looked for. So let me help you on this one:
Any resources about Googles APIs can be found here:
https://developers.google.com/
If you look for an implementation many of them can be found on Github:
https://github.com/googleapis
As for why? Well, I guess because it's an already existing platform allowing for easy collaboration. I myself have some fixes in now official releases of some of the APIs - although only very minor ones so not really worth noting. I guess it's because so easy to be part of: If you found the API you have an issue with or even some fixes you can just fork the tree - implement your fixes - and file a pull request to get it merged into the official releases.
Also: When you want to ask something you'll find yourself on the StackOverflow platform - cause Google chose it over providing their own forum/board/what ever.
Is it because Google couldn't had provided it all on their own? No, I don't think so, but rather because re-use of already existing platforms. Someone here remember Google Code? Never got that big because other platforms existed before it and are way more popular. It's quite easy to think about why someone thought: Ok, just lets use what all those devs already use ...