That must be because It’s way easier:
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To export attachments, you just search for items that have them and download them in some directory structure.
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To restore them you need to come up with a way to pair each attachment to the item it belonged to when it was exported; which is not so straightforward.
The scripts that I use (and wrote myself) try to do that. They are written in bash, so they might be a bit more difficult to use on Windows.
But, in any case, it should be definitely easier for everyone once Bitwarden provides a direct way to export attachments from their client(s). As @DenBesten pointed out.