In the case of collaborations between separate companies, secrecy becomes a problem because inevitably, details that are held close and not communicated turn into stumbling blocks. For a collaboration between Nvidia and Microsoft for instance, the teams at Microsoft involved with the project would need the same level of access to Nvidia's half as Nvidia's employees do in order to be as effective as Apple's internal teams have been. This isn't impossible of course, but historically Nvidia is not inclined to do things like this.