Why is iPod Shuffle's 'voice over' feature not available on the iPhone?

Just wonderin' -- the iPod Shuffle has a really cool feature where a synthesized voice will announce the name of a song over the earphones upon request. Super useful feature on any iPod, right? Then how come is it not available on the far more powerful and expensive iPhone? Is there a technical reason or a marketing reason or some other mysterious reason?
Posted on July 15, 2010 and filed under Technology.