Your body is a conservative mechanism. This means that it will adapt to be better at what you are making it do, and will not maintain things that you are not using. Why would your body waste calories and energy building and maintaining muscles that are never used?
Although we know that having a fit body helps you to live longer, lifespan is not what your biology is centered around. Your body was created by evolution, and the driving force behind evolution is reproduction. Allowing unused muscles to deteriorate saves energy in the short term, because a big muscle burns extra calories even when it is at rest. Survival in the short term is important to being able to reproduce, so evolution favored that. Because human fertility diminishes later in life, a longer lifespan is not an evolutionary advantage.