The battery time estimates are based on certain usage patterns, not 100% utilization of power-hungry apps. A battery that can last "36 hours" can offer more real-world usage per day than one that only lasts for 24.

Edit: Looks like the only estimates Apple provides any more are video playback hours, which is much more power-efficient than general app usage and internet browsing.

Do you really think they couldn't?

The magsafe battery thing is actually pretty neat, but not using it to instead have a thinner phone feels pointless because you still have the giant bulge from the camera.