In many countries nowadays, especially Western countries, we live in abundance. There’s food everywhere, and we seldom die of starvation.
However, our genetics over the last 10,000 years has remained pretty much the same.
Hence, the majority of us will tend to store excess fat when we overeat, and not so much muscle.
What’s worse, because it is seen as inefficient to carry excess amounts of skeletal muscle around, the body would more readily break down this excess muscle and keep the fat as opposed to vice versa.