Apple brought chip design in house because there wasn't a suitable SoC in the market.
And apparently, they also felt there wasn't a suitable ARM core design in the market, as they designed the ARM core for the A6 in house as well.

Because Intel's isn't used to providing foundry service, porting a design into Intel's process and getting good results requires a lot of effort, much more than porting from one foundry to another.
In order to make sense, it would have to be a long term commitment between Apple and Intel, and it would be for large productions.