It’s basically because: “The term (monkey) is used to describe items that are smaller than normal size”
https://forums.sailboatowners.com/threads/monkey.39008/ -he said the cabin appeared to be an afterthought like a box just stuck on
Monkey spar - a smaller spar
Sailing Terms Monkey Deck A false deck built over a permanent deck. Often used in the bow of larger sailing ships, forward of the anchor windlass and provides a working platform around the portion of the bowsprit as it attaches to the ship.
Monkey Island was located about 3/4th way up the main mast on a few ships, and sailors used it for solar and stellar observations. - Marine Insight
monkey bridge
A high platform above the wheelhouse offering better visibility to the operator while maneuvering. - Wikipedia Glossary of Nautical terms