Very few games this generation have been native 1080P but as both consoles came out when 720P was just taking off that's understandable.
Multiplatform games are likely to run 720P on Xbox One, developers will probably program games at 720P for Xbox One and then use the PS4's extra GPU power to get an almost free resolution increase.
PC's are highly inefficient compared to consoles, developers can't communicate directly with the hardware that's why AMD Mantle is being promoted, plus the highest settings on PC (Ultra etc) aren't really optimized and are just there to offer a little bit more visual difference even if the performance impact is huge.