The current implementation is so that you can `<script type="importmap" src="path/to/your/importmap.json"></script>`. JSON is a well-known on-disk data format with lots of tools that support working with it and manipulating it (including plenty JS itself including natively in the browser).

You can even `<script type="importmap" src="https://your.fave.cdn.example.org/shared-importmap.json"></s...` and share an importmap across multiple applications/projects.