Why are wp_terms and wp_termtaxonomy separate tables?

At least now there can be terms of the same name in different taxonomies, but they remain separate terms. Taxonomies are hierarchical. A single term in multiple taxonomies cannot know what its parent is, breaking the hierarchical concept.

It’s true the terms and term_taxonomy table could have been combined. It was decided early on they be separate. I’m guessing a different schema was initially intended that made separate tables logical. Even though that schema is no longer used, the tables remain separate for reverse compatibility purposes. It may be more confusing for us humans, but SQL doesn’t really care. One to one tables is basic stuff for SQL.

I see, thank you.