I am genuinely curious what are the specific issues which necessitate deleting all modules? Colleagues across operating systems with native modules? Old node versions?
Why? Because installing the dependencies they specify and running babel (implicitly at the latest version) does not have reproducible results. It's that simple. package-lock.json is specifically supposed to help with this, but in some cases, it just doesn't, and it's entirely possible, and common(!) to have two sets of package.json files produce differing package-lock.json files.
They even have a warning below that can still occur even if you already have the dependencies specified above installed:
> If you see an error message saying “You have mistakenly installed the babel package”, you might have missed the previous step. Perform it in the same folder, and then try again.
[1]: https://reactjs.org/docs/add-react-to-a-website.html
[2]: https://reactjs.org/docs/add-react-to-a-website.html#add-jsx...