I know nothing about how processors work so tell me if this even makes sense. But other than the time and money is there any reason that say a Google can’t create their 100% from scratch CPU and instruction set? Same for apple and all the others?
What's more, as far as I know, it's not their expertise area, they outsource this kind of work (architecture and cpu design), so if they had to develop their own architecture or CPU, relying on work from other companies with this expertise would probably lead to better results (for cheaper), for instance for a very specialized kind of work that would be more efficient with a specialized architecture.
And yes, that would probably be very time-consuming and cost a lost of money and that alone is probably a big reason for not doing it, especially if there is no clear need for it.
They probably would if they both had the expertise to do so and they were in a situation where developing a new architecture and CPU from scratch would allow them to save a lot of money.
For general workloads, they are probably just better off using a widespread architecture to leverage existing tools and ecosystems.
And maybe they actually do for internal stuff without us knowing, but that does not seem very likely.