Vascular bundles in monocotyledons are considered closed because

Vascular bundles of monocots are described as closed, as they do not have cambium in them.
(1) Vessels always have perforations.
(2) In monocot stems, vascular bundles have outer phloem and inner xylem.
(3) Since fibrous bundle sheath is present, vascular bundles of monocot stem are described as fi bro-vascular bundles.