Not a dumb question at all. My uneducated guess is that it is a question of size and compactness.
If you hit a tank with an armor piercing ordinance, let's say you get a 1" hole in the armor and 20 cubic foot fire ball/shrapnel on the backside. The tank is pretty much crew space, engine, fuel and ammo. Chances are pretty good you will hit something that will take the tank out of the battle.
If you hit a ship with the same ordinance, with the same hole and fireball, it's more likely you will take out space. Maybe a couple of sailors. Maybe some important system. But if you don't hit fuel or ammo, that ship is still in the battle. Even if you hit it below the water line, they will close off the segmented hatch and patch it later. If you've ever watched footage of WW2 naval battles, those ships can take a ton of damage and stay in the fight. Until, suddenly, a lucky hit gets the fuel or ammo, and then it's over. Which is why fuel and ammo are heavily armored.