Years ago, when troubleshooting performance, I stumbled across this Microsoft documentation on parallel query processing that says:
Certain types of statements cannot be processed in parallel unless they contain clauses, however. For example, UPDATE, INSERT, and DELETE are not normally processed in parallel even if the related query meets the criteria. But if the UPDATE or DELETE statements contain a WHERE clause, or an INSERT statement contains a SELECT clause, WHERE and SELECT can be executed in parallel. Changes are applied serially to the database in these cases.
Thank goodness I bookmarked it, because I moved on with my life