I may be reiterating already clear (as Claret)
responses, but I thought I would have a go at this...........
The power curve of a turbine engine ensures that despite the power loss at altitude due to the lower air density, said power loss is substantially less than the dramatic drag reduction of the airframe at altitude - also due to the lower air density.
More available thrust VS less airframe drag = mucho go-o
It's almost like getting a free lunch.