AMD was originally planning to have an ARM CPU that would be compatible with its AM4 socket. I believe their first customer was Amazon but Amazon dropped them for an in-house design that became Graviton due to performance reasons. Given AMD was not in a good financial state at the time it made sense to concentrate on Zen. It would probably be a very different world if AMD had managed to debut a 'Zen' inspired ARM CPU alongside its first batch of Zen processors.