> Why pay for a service, and have the little mental overhead of it existing, when I’ve duplicated it on my own site?

For me, because Pinboard has IFTTT integration where I can feed a whole bunch of things[0] into it automagically. None of the self-hosted services has IFTTT integration or, as far as I've seen thus far, webhook functionality I could feed from IFTTT[1].

I've got a self-made feed from Pinboard to a self-hosted linkhut which works fine (absent the archiving capability but I've never really used that anyway.)

[0] Although sadly that list does keep shrinking as arseholes close off / commercialise their APIs.

[1] A REST API doesn't work because they rely on setting the 'Authorisation' header which, AFAIK, isn't supported in IFTTT's webhook.