Link to Original Source
The rest of the world gets Android code when Google feels like releasing it.
The open source world has TONS of excellent APIs, no sense in not using them.
What sort of APIs are your talking about? Much of what existed pre Android was software built primarily for desktop use, disregarding things like battery and memory usage and implementing a feature set far greater then what is needed on a phone - like the X window system. Nokia tried porting a true GNU system to a phone with Maemo, but it looks like it wasn't much of a success.