"Why don't they put thrust indicators in multi engined aeroplanes alongside the power levers?"
For the same reason you don't have a tachometer next to the accelerator pedal of your car, it is not where you (should) be looking when that information is important to you. From memory, I believe that a contributing factor to the mis-identification was that the engine display was wired incorrectly, telling the crew that the good engine was the failing one. If that was the case, your proposed solution would not have helped.
Having said that, I think that the aircraft in question had that LCD combined engine display that I found extremely poor compared to the old dials or the NG display.