I have written my own zip, deflate, and huffman coding - although the latter two were "just for fun". But I would definitely consider writing ad-hoc zip logic in real software, if I couldn't pull in a library for whatever reason. This isn't just a hypothetical, it happens a lot - there are many independent ZIP implementations in the wild, for better or for worse.

You're right to call out security though, because the multiple implementations cause security issues where they disagree, my favorite example being https://bugzilla.mozilla.org/show_bug.cgi?id=1534483 . Although arguably this is a symptom of ZIP being a poorly thought out file format (too many ambiguous edge-cases), rather than a symptom of it being easy to implement.