In short, because the upper half spends more time with the hottest gasses than the lower half. The greatest heat loads are where the majority of combustion takes place, the highest combustion temperatures are reached, and where the hot gasses are in contact with the cylinder wall for the longest period of time, which would be in the upper half of the cylinder. By the time the combustion gases contact the lower part of the cylinder they have expanded and cooled considerably therefore the lower cylinder walls absorb far less heat. Cooling from scavenge air is most likely negligible. From air cooled lawnmowers to liquid cooled automotive engines you'll notice the cylinder head and the upper cylinder half have the largest/most cooling fins or have the greatest internal cooling water flow compared to the rest of the engine.
Air from the turbocharger is cooled in the charge air cooler before it enters the air receiver space. Typically this reaches 180+ degrees after the TC and is cooled to 50 degrees.
Ellen, I'm not sure who told you such a thing, but consider these facts. Low speed marine diesel engines are some of the most efficient internal combustion engines in the world, meaning that they extract the maximum amount of power from the minimum amount of fuel. In doing so, the achieve a level of combustion that reduced the amount of unburned hydrocarbons that exit the engine as exhaust gas. when you consider that a large jet aircraft consumes as much fuel per hour of operation as a large ship, I think that you can readily see that there is likely significantly more fuel being burned every day by commercial aircraft world wide than by ships. If you are interested in actually finding the answer to your question, I would be happy to point you towards primary data sources.
