If we truly evolved from monkeys, then why did we lose our mass of muscle fibers?

Muscles are expensive, takes a lot of energy to grow and maintain. If tool usage can negate the need for strength then tool users with weaker muscles will be more fit because they require less food.

Think of it this way a lion can only run for say 5 minutes (for the sake of this example) if a gazelle can run longer than 5 minutes it can survive, now imagine a gazelle that can run for 10 minutes, when the average is 6 minutes... that's more muscle, that's more food, for the same survival advantage of a 6 minute run... not vary efficient.

with a tool you can essentially multiply the amount of force your muscles put out so many times that you don't really need to exert yourself to get through the day.

Sorry. Dont understand your question. Clearly we did not need the muscle mass if it was not selected for. Evolution does not work towards a predefined goal. It just depends on which animals that breed have which trait. If the animal survives those traits will be passed on. Evolution does not say "ooh, that is good, I'll keep that but I dont need those so I will dump them".

It is simply natural selection - that was Darwin's great idea - not evolution as is frequently erroneously believed.

While chimps can sprint faster than humans, humans are considerably superior when it comes to long distance endurance. We also swim better.

Always remember. We did NOT evolve from monkeys. Humans and monkeys evolved from the same ape like ancestor.