Why do jet planes with multiple engines have separate throttle levers for each engine?
All multi-engine airplanes, whether jet or prop driven have separate throttle controls for each engine. There are good reasons for this:


If there is a problem with one engine you can separately control it for troubleshooting purposes
If one engine isn't developing the right amount of power you can independently increase the other engine(s) to compensate
If you have a control failure of some kind you can use differential thrust to steer the airplane, this has been done more than once, the example that comes immediately to mind is United 232