1) All materials undergo fatigue from stresses. Each and every flex wears on them.
2) Unless you're a delicate rider, you'll apply shocks to your board over and above simple fatigue occasionally. Those add up too.
3) If boards were totally and completely waterproof and watertight, it wouldn't matter. But over time seams will loosen, microfractures will form, and moisture will get into the core. I've had to replace ceiling rafters that got wet, so I know what moisture does to wood.
4) Volatiles gradually evaporate, which will result in man-made materials gradually becoming brittle.
5) Two words: Ultra-Violet light.
Can't say how much
effect all this will have, but pretty sure it'll have some