Why not? The harm is to heat dissipation at high clock rates and voltages -- basically to overclockers. The targeted clocks at the performance points they are aiming at don't have any trouble staying in reasonable temperature ranges, so why bother?

Lots of this is market based, not technical. The only competitors to Ivy Bridge are from AMD, and frankly don't compete well. There's simply no pressure to produce a "pull out all the stops" max performance chip. They have a comfortable margin even with comparatively poor heat sink designs, so they might as well save the nickel.