From the description of flatten/2 :

"Ending up needing flatten/2 often indicates, like append/3 for appending two lists, a bad design."

append/3 does not surprising or optimized either. Clicking on the "source code" button on the top left reveals that it does exactly what you would do when coding append by hand: recurse down the backbone of List1 until _[]_ has been reached, thus copying it until the fresh variable that is the tail of the copy can be unified with List2.