Why many NATO patrol ships retain 20 mm guns

Why are so many NATO offshore patrol vessels armed with 20 mm guns?

Because 20 mm is cheap and often proportionate to the patrol vessel's size, operating area, and expected threats.

NATO navies often fit 20 mm guns to offshore patrol vessels because the weapon is inexpensive to buy, crew, and support. The size of the particular OPV, where it operates, and the threats it is likely to encounter matter as much as calibre. A small patrol ship in a low-threat area gains little from a costly weapon intended for another mission. Patrol-ship armament should remain proportionate to the work assigned to the vessel.