They're not the same thing which is why they're not as useful.

Magnet links apply to an entire collection of files, as snapshotted by the creator. There is no way to pick a single file and refer to it, immutably. It's all or nothing. Additionally, metadata besides the file contents will change the hash. This includes filenames, directory structure and piece length (chosen at snapshot time, by the creator).

All of these limitations are enough in practice to make magnets useless for widespread per-file distribution.