Why is UPDATE with column-list syntax not implemented

9.3 documentation says:

According to the standard, the column-list syntax should allow a list of
columns to be assigned from a single row-valued expression, such as a
sub-select:

UPDATE accounts SET (contact_last_name, contact_first_name) =
(SELECT last_name, first_name FROM salesmen
WHERE salesmen.id = accounts.sales_id);
This is not currently implemented — the source must be a list of independent
expressions.

Why is this not implemented? Is it considered inconvenient to use, or
difficult to implement. or not important enough, or some other reason?

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