I'm not sure that's true. Intel has managed to have a higher performance process than their competitors in part by accepting more restrictive design rules. Because their architects can work hand-in-glove with their process engineers as a new node comes up they can have two-way feedback on this and to some extent design their process around their architecture's requirements. But it's very hard for third parties to work with their main process. They do have a separate relaxed rules process for use when acting as a foundry.

Pursuing the foundry route seriously would have meant Intel giving up one of its largest advantages.