For the Metabase core teams view on why they just didn't add joins front and center see these blog posts:
So, you can still do joins inside Metabase - it's just they sort-of hid it away for most users as "native SQL questions"... in particular I'd say the "Questions in Questions" approach combined with SQL knowledgeable users doing fancy stuff as pre-cooked qustions to build on for others is powerful.
The thread below may also shine some light on the topic - and how some of the (somewhat related) related PK-FK mapping shines trough in the UI
I hope that answers some of what you were asking?