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Because it isn't metadata as calibre understands it.
mobi-asin is automatically picked up from the exth 113 (ASIN) header, which is usually used to show a link to the Amazon store. If that header has content in it, Kindles will stop showing the "Personal" banner.
Changing it requiires conversion in calibre (there are MOBI output options that control its creation) or modifying the exth headers using something like Java Mobi Metadata Editor.
ebook-meta cannot get rid of this non-standard mobi-specific metadata field, but it can read and import it into the "identifiers" metadata, which is what is confusing you.
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