OP isn't asking about automatic shifting.
He is asking why the bike can't shift to the next higher or lower gear with one shifter instead of two, why the bike can't figure out on it's own whether the front, rear or both derailleurs need to be actuated to get to the next higher/lower gear.
This already exists. Shimano Synchro Shift.
Synchro Shift is not automatic shifting but linear shifting - you go up and down the gears and the bike decides whether to shift front or back derailleurs for you.
-Tim-
Last edited by TimothyH; 02-05-17 at 12:38 PM.