Err, because it works?

A few grains of rice in the salt cellar to stop the salt getting soggy works too you know.

It is true that rice, being hygroscopic, soaks in moisture from the surrounding air. This starts taking place long before you put a wet phone in it. Which is why it is imperative to keep your rice in a closed container. It does not need to be hermetic, even the plastic bag it came in from the shop is good enough. But it is never fully saturated even after months, as evidenced by how much more water it can take in when cooked.