Because a venn diagram does not describe join mechanics well. A venn diagram does however describes the UNION, INTERSECT, EXCEPT part of sql.

https://blog.jooq.org/say-no-to-venn-diagrams-when-explainin...

And more meta, it is an innocent slightly incorrect statement, stuff like that should not be down voted, reply with a correction. Save down votes for outright malicious posts.