Why German and Japanese smaller guns lacked AP shells

Why did German and Japanese 4-inch and 5-inch guns lack armour-piercing or semi-armour-piercing shells?

The likely explanation is that expected targets favoured high-explosive and anti-aircraft ammunition, leaving little incentive to fund a separate armour-piercing shell.

The suggested explanation follows the expected targets of the guns. Destroyers and other light vessels usually carry little armour, so a dedicated armour-piercing projectile offers limited benefit. High-explosive and anti-aircraft shells address the surface and air targets those weapons were most likely to engage. Developing another shell type would also consume researchers and money that even major states do not possess in unlimited quantities. This is an interpretation of the likely requirement, without a specific procurement decision identified.