I was thinking maybe because the ocean is a lot flatter than a lot of terrain but there’s also large waves and storms and sea. I also can’t imagine that something like an aircraft carrier would be significantly less complicated or difficult to build than an equally sized land vehicle.
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You’ve probably heard an explanation for why a person can survive lying on a bed of nails that runs a bit like this: their weight is spread out over many many points, which reduces the weight pushing on any obe point. That’s almost enough info already. The thing is, they’re not infinitely small points, the tip of the nail has some non-zero area, even if it’s 1 mm² the amount of weight (or force) applied to the tip of each nail is (on average) the entire weight of the person divided by the total number of nails.
So let me put some numbers to it. A 150 lb person manages to spread their weight evenly over 1000 nails. That means 0.15 lbs is pressing on each nail. I’m sure you can already imagine that if you pressed a nail into yourself with 0.15 lbs (0.667 N or the same force as 68 g would experience due to gravity. So imagine a 68 g nail sitting point down in your hand. According to Google, the area of the tip of the nail is 1/100th of 1 in². That’s actually closer to 6 mm² than my original estimate, but here’s the key part. Pressure is force divided by area. So our force is 0.15 lb and our area is 0.01 square inches. That’s 15 pounds per square inch (psi). Which is actually really close to 1 atmosphere of pressure. The air around you pushes you with 14.8 psi. Google says 100 psi is needed to puncture skin.
So, now that I’ve become far too long-winded, let me bring it back to ships. Water is a fluid. The water pushes on you, if you go a little deeper, the weight of the water above pushes on the water below, which pushes on you. So the deeper you go, the higher the pressure. But more importantly, it’s pushing everywhere it touches, all at once. It’s like a bed of nails, but the nails are as closely packed as molecules. Every square inch is being pressed on by quadrillions of “nails”
When people talk about the weight of the vehicle being supported, this is what they mean. The ocean is just the world’s best bed of nails. It distributes the weight over the entire surface of the ship’s hull. And it’s not perfect evenly distributed (deeper bits of the hull experience higher pressure) but there are no gaps between nails, so it’s more like just lying in a bed, than a bed of nails. Land vehicles aren’t vehicles unless they move, and it’s impractical to make the entire bottom of the vehicle 1 giant track. Which means the weight is being spread out over a smaller surface area, meaning higher pressure. This means stronger materials are required. Usually bigger thickness axels and stuff, meaning it’s heavier, meaning stronger materials. Either the embiggening cycle never stops or it does, but it costs so much that it’s no longer feasible.
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