As mentioned, the rollers leave the factory with no grooves.
When on location in an arcade, these machines were played by thousands of sweaty palmed teenagers. Sweat is essentially a combination of warm water and salts, which can contribute to rusting/softening of metal. I figure the combination of the mechanical action of the ball (which isn't that soft) and the fresh supply of sweat breaks down the roller over time.
It's still quite amazing to see how much of a groove the rollers get though.. I'm betting the ball itself gets worn down too, but you can't see it so much because the wear is uniform.