[znwu]

Mainly because they can. They already have excellent team & experience in designing low power cores from the ground up, and also they have their great LLVM team for the compiler. Core plus compiler, that's all you need for a in-house processor. There are very few companies with this kind of combinations, and those who have them all chose to build their own hardware stack.

When you have the hammer, it becomes a crime to not hit the nail. Especially so when you find out you have a better hammer than your hammer contractor.