Ooh, ooh, I'm an engineer (soon)! And I'll tell you what: the chain (pins) wears by erosion, eventually causing shear. The erosion rate is proportional to the force exerted on the chain. While accelerating on a properly tensioned chain, there is much force, but once pedaling solely to maintain speed, most of the force is gone, letting the chain relax.

On an overtightened chain when the rider gets to speed, the chain is still wearing as if the rider is accelerating as there is too much tension. Therefore, the chain wears far more quickly than it should.

While loose chains are a bad thing, overtightened chains are far worse for durability.