I think there are two factors preventing mobile from being a force to drive performance optimizations.
One, phone OSes are being designed for single-tasked use. Outside of alarms and notifications in the background (which tend to be routed through a common service), the user can see just one app at a time, and mobile OSes actively restrict background activity of other apps. So every application can get away with the assumption that it's the sole owner of the phone's resources.
Two, given the above, the most noticeable problem is now power usage. As Moore's law has all but evaporated for single-threaded performance, hardware is now being upgraded for multicore and (important here) power performance. So apps can get away with poor engineering, because every new generation of smartphones has a more power-efficient CPU, so the lifetime on single charge doesn't degrade.
