It's not just the ISA that you get when you license from ARM: it's the entire HDL source. If you go even further and get an architecture license, you also get a

It's not just the ISA that you get when you license from ARM: it's the entire HDL source. If you go even further and get an architecture license, you also get a whole bunch of fundamental technical documents and a test suite. This saves a ton of time. There's the issue of software compatibility. Writing a port of Linux—kernel and userland—is a ton of work. And if you have other chips on board, such as a GPU, the only drivers available may be ARM-only binary blobs. There might be an entire ecosystem of ARM software you have to keep supporting (which is one reason why Apple doesn't [yet] design their own ISA). RISC-V has a small chance of commoditizing some of this stuff someday, but not yet.