Part of that is almost certainly because it is far, far easier to explain how to get started with create-react-app then it would be to explain the various alternative approaches, especially considering how inexperienced on average the kind of people who are looking up how to set up a react project must be (professional developers needing the information for their actual work probably constitute a small minority), even if those alternative approaches themselves are not complicated at all to those who have the prerequisite knowledge to understand how a library like react would be incorporated into their particular setup.