And quite often, those players were seldom if ever played. It's bacteria and moisture, with heat which gradually destroys leather. In the midwestern plains where lots of corn is grown, the air is full of mold and food for bacteria to thrive on, so the tanning acids will not fully poison all the bacterial growth, and if the player is played a lot, it will quickly contain plenty of food and bacteria to sustain the colony for years to come. If the player then goes dormant for many years without being constantly played, the bacteria are able to work faster. But even so, the midwestern plain states and agricultural areas where pollen and mold counts are high seem to rot the valves more quickly.
Old leather leaks more than that, initially.
If you would like to check this out for yourself, take a plastic film can and punch a hole in the lid representing the inside valve plate hole. Apply a suction to the other bubble jar tube with the short tube above the water. The bubbles you see are representative of air leakage. By changing to different leather, you should be able to tell a difference. The Orchestrion Builder's Manual and Pneumatics Handbook has a large appendix in the back covering all these tests and much more.