Enemy ships set the harder requirement for naval guns

Why were naval guns not designed mainly for shore bombardment when ships fired so many rounds at land targets?

Enemy warships imposed the more dangerous requirement because a poorly matched naval battery could get its own ship sunk.

Shore bombardment may consume a large share of a warship's ammunition, but enemy ships remain the harder design case. A gun optimized too narrowly for land targets could leave the ship unable to defeat an opponent at sea. Shore targets do not normally return fire with the same immediate ship-killing effect. The battery therefore has to meet the naval combat requirement first and perform bombardment with that capability.