This is because fuel injector nozzles become worn over time. As the nozzles wear the fuel pressure reduces, and the fuel then doesn't atomise quite as well leading to reduced combustion efficiency. A new set if injector nozzles will give the engine a new lease of life.

Pumps don't create pressure, they create flow. Opposition to flow creates pressure, and that's why the condition of the injector nozzles are so important.....