Animals like gorillas seem to have a physical fitness and muscle mass that don't depend as strongly on how much exercise they get, compared to humans. E.g. gorillas living in the wild sleep and rest a lot. While they do get quite a bit of exercise (a lot more than the average human), they are not going to exert themselves much more than is necessary. So, they push themselves a lot less than people who exercise very hard. Yet the average gorilla would easily outperform even a well trained human being in bodyweight exercises.
So, there seems to be something fundamentally different about maintaining physical fitness in humans compared to gorillas.