The optimizer currently doesn't unnest scalar subqueries in the select clause, even when feasible.
Here's a list of such cases:
Correlated Subqueries in the SELECT Clause - All-round Database Topics (0 Bytes)https://nenadnoveljic.com/blog/correlated-subqueries-in-the-select-clause-2/
Such unnested subqueries must be executed for every row in the driving rowset.