hmm... leaves me speechless..
You'd have best remained speechless, given your poor understanding of turbine engine efficiency, and the purpose for flying high.
Simply put, a turbine engine can't be run at it's most efficient state at lower altitudes, due to excess thrust and excess airspeed. Only when we are required to run at higher power settings at higher altitudes are we able to operate the powerplant at it's most efficient state.
During cruise we would like the aircraft to be faster and engine to be efficient.
Obviously.
They don't fly high they fly at an optimum altitude. So if the interviewer asks you this questions and you start with politely rephrasing the question to "Why do jet aircrafts have an optimum cruise altitude higher than turboprop or piston prop aircrafts?", you may win the interviewer without even answering.
You throw out this rubbish and then go on to whine about "circumlocution in its worst form?" The answer to why a turbine engine is best operated highest...is "because its' optimum altitude is higher than turboprop or piston prop aircrafts?" Not much of an answer. You might as well go with the Acing the Technical Interview responses. You'll get nearly as much mileage.
Optimum altitude is a function of aircraft weight, ambient temperature, desired cruise speed, etc.