The ISA is just a small part of what's required. All the other interfaces are just as important such as pinouts, memory mappings, memory interface, bus support and so on.

There's very few SoCs from different vendors that are interchangeable even among the ones that uses an open ISA. This requires you to either build everyhing from scratch including the CPU, or to use more generic and perhaps interchangeable CPU or microcontrollers and rather push the special components to other chips instead of having those components on a SoC - which gains you nothing.

Meaning the value of using an open ISA is miniscule as long as the code is available and easily translated to another ISA by a compiler.