And that's the reason why we see so many consoles ports nowadays. Xbox uses DX and you can easily port it to the PC. No sane company will start a multi port game with an low level API in the mind. Optimizing only starts after the port to the target plattform
Game engines do console specific optimizations much closer to the hardware than DX allows. This makes sense because:
1. The hardware wont up and change on them overnight.
2. The hardware wont up and change on them overnight for the better so if you want improvements you're going to have to squeeze it out.
3. Consoles are a large part of the market.
4. The option to actually get more access to the hardware is there.
5. Some other developer will do it and you'll be behind.
6. Investments made in optimizing said engine can be leveraged and amortized over the life of a console.
So of course they use DX, because a large part of the market can only use DX, or OGL but whatever. Again, that does not mean they do not use other APIs or that there is this humongous barrier to supporting more than one path. As far as I'm concerned, the whole console model encourages exploiting as much of the hardware as possible and when they do put in the effort to make these things work as best as they can, mantle should be there to make it as effortless as possible for these optimizations to migrate to other platforms with GCN.