Trees are often considered too diverse to include even in the standard library, let alone as a primitive. Even Python doesn't have trees in the standard library. I'm sure it's been proposed and rejected at some point
I hate to be a broken record, but this again goes back to the assumption of a specific sequential computation model. Modelling relations as in SQL automatically supports for N-ary child relations. There are other computation models! eg. logic, relational, etc.
Let's be practical about it: the reason you might want to say map_tree is so it could potentially be optimized to run more quickly, e.g. via automatic parallelization.
But to even parallelize a tree read operation we'd have to pull in other threads, split work, we'd want metadata on each node (child counts), and at the end of the day within each worker you'd still have a serial execution order.
For map_tree to have practical benefit your language would need to make a ton of opinionated choices about trees and parallel programming. Feasible? Yes. Worth making it a basic primitive? Well, even LISPs don't offer map_tree, despite building the whole language on top of car/cdr.