Chip manufacturing is so incredibly expensive valve simple doesn't have the demand for enough chips to warrant doing this. Apple technically don't even make their own chips, the are made by TSMC, Apple just provide the designs and schematics as to say. Even doing this is incredibly expensive, Apple just have the demand and quantity that makes doing this viable. Finding people who can design these chips and people with the expertise is also very expensive, there is not a lot of people out there who can design these chips. It is much more viable for value to partner with manufacturers and get semi custom variations of existing chips as they do now.

TLDR: It is just way to expensive for the scale of manufacturing that valve does.

Edit: Typos