Why term_id is not indexed in wp_term_taxonomy table?

Every category or tag (i.e. electronics) is stored as term in wp_terms table which is indexed with name and slug, and its relationship with taxonomy is stored in wp_term_taxonomy table which is indexed as taxonomy and term_taxonomy_id.

I can find term_id from wp_terms table using name efficiently because it is indexed with name, but then I have to use term_id further to find term_taxonomy_id from wp_term_taxonomy table, but it is indexed as taxonomy but not term_id, and then further I’ll use term_taxonomy_id to find object_id (posts) from wp_term_relationship table.

Question is, why wp_term_taxonomy table is not indexed with term_id which is important in operations like finding posts related to a particular category or tag or anything else?

A single term can, theoretically, be present in multiple taxonomies (and have a different description in each). That’s where term_taxonomy_id comes into play, referencing a single term within a single taxonomy.

In practice, WP tries to avoid that, and will create a term with the same name and a -2 (iirc) suffix on the slug if a term with the same slug already exists in a different taxonomy.