why is default required when importing json files in js code?

This happens because the import * as line imports an_entire_modules_contents where cause the absence of an explicit export default the default property coincides with the content of your json file. If you want the data variable contains your json file you can use instead:

import {default as data} from "./data.json";
console.log(data); //<-- it will print the content of your json file

Or the simple shorthand below:

import data from "./data.json";