Building from scratch isn't "easy". They still buy a lot of their components off the shelf so they do need to interact with vendors so they can benefit from the

Building from scratch isn't "easy". They still buy a lot of their components off the shelf so they do need to interact with vendors so they can benefit from the economies of scale on the PC side of things. Yes, they can optimize a lot of pipelines but this goes both ways. E.g. they had lightning connector which was good. USB-C beat the pants out of it and now has a much better eco-system. Apple is stuck with their sub-par connector for phones and an unclear strategy (USB-C on macs/iPads but not on phones and low end iPads).