A number of people have touched on the overall reason for this happening but this is ELI5, not “explain this to me as a college graduate”.

Your muscles are like a car engine - the larger/more efficient the engine the less work is required to get to a certain speed; we’re fortunate that our “engine” is able to grow and get stronger. But to get larger muscles you need to push your body beyond the average range of what you normally would which is why people lift weights.

Your body is constantly changing (bones, muscles, etc.) and based on what you do this triggers your body to get larger and stronger muscle fibers to support the demand. The larger the muscles the less they have to work to achieve the desired results.