They could,. Unfortunately, it requires a huge investment and needs a lot of highly skilled staff, who are currently working on its mainstream product. Also, when you've done your new chip design, you still have no hardware devices and no software to run on them.

It's taken roughly 40 years to get x86 to its current position, and almost as long to get ARM to its current position. For how many years would you have to invest in a new architecture (ie lose money) before you could compete with them?

AMD has just done a new chip design (Zen) but it's x86-compatible because that's where the software is....