Re: Why is there a Sort after an Index Only Scan?

Quick(?) question... why is there a Sort node after an Index Only Scan?
Shouldn't the index already spit out sorted tuples?

CREATE INDEX ON orders_test(shipping_date, order_id);

EXPLAIN ANALYZE SELECT
FROM orders_test
WHERE TRUE
AND shipping_date >= '2022-05-01'
AND shipping_date <= '2022-05-01'
ORDER BY order_id
LIMIT 50;