Look at it from a theoretical perspective and it’s quite simple. All those “conventional 150mm howitzers” you’re building for 4 IPCs don’t have the range to retailate against weaponry of cruiser caliber or above. So they’ll sit there on the coast under bombardment and like it!
If you don’t like it, then your answer is to create a house rule for a coastal defense / bunker fortification that can be built 
I have been fooling around with a rather simple fortification/blockhaus rule:
limit one per territory
10 IPCs
When attacked via land or by amphibious assault , rolls ONE retaliatory bombardment die at 4 or less.
Roll is made after any incoming bombardment fire from attacking vessels and you take an applicable hits. Any casualty you inflict is immediately selected and removed from the board by the attacker. The blockhaus DOESN’T roll a defense die in combat from then on.
Additionally, any hits you take during the rest of the battl against infantry or artillery in that territory are negated by saving throw if you roll a 6.
If the territory is captured, the fortification/blockhaus is removed from play.
