This question is an exact duplicate of:
Tag excerpts from 'obligatory' tags (e.g. discussion) and 'moderation' tags (e.g. status-completed) on per-site metas do not appear in the all tags page. However, in Meta Stack Exchange they do (see picture below).
This happens probably because while in Meta SE the excerpts have a history (were edited at least once) on the per-site metas which I cited as examples still have the system's default excerpt.
Nonetheless, I think we should take advantage that there are excerpts for such tags an enable them in the 'all tags' page.