It's not just patents that matter though, look at the x86/AMD64 cross-licensing between Intel and AMD64 - both are licensed to produce x86 and AMD64 compatible processors and besides VIA (I'm not exactly sure of the terms with VIA but they've been in the game a long time and don't directly compete with Intel), no-one else. Nvidia tried to get a license from Intel a few years ago, likely for Denver, but wasn't able to.
Unless patents are considered critical and hence given FRAND status, companies aren't necessarily obliged to license technology.