Why can't humans fly around by using wings?

A bird can fly because its wingspan and the wing muscle strength are in balance with its body size. It has a lightweight skeleton with hollow bones, which puts a smaller load on its wings. A bird also has air sacs connected to its lungs, which makes it even lighter and allows for easy passage of air through its lungs during flight.

because that's not how we evolved

The principal drawback to humans flying around with wings is that we do not have wings.

Because we have not evolved the ability to fly. Flying is dangerous and costs a lot of energy. If there is no advantage in flying, then there is no incentive to try to fly. Unless one tries to fly, any mutation that helps one fly will not become advantageous, and will likely be lost through chance. If an animal tries to fly, then any chance mutation that helps it fly will become advantageous and it will likely be retained and even spread as this individual will have more descendants that carry the same mutations. That is what biologists mean when they say that behavior often drives evolution. IOW, the body and the genes cannot anticipate future needs. They cannot evolve features that will help a body fly in advance of an animal trying to fly.

Biophysicists point out that the evolution of flight is implausible in a large, ground dwelling organism, and humans are large ground dwelling organisms. Instead, flight most likely evolve in small arboreal animals, especially among animals that glide. That may be why bats evolved flight, as they are so small, and their ancestor was a small animal that resembles a shrew. For humans to have evolved flight, our ancestor would have had to evolve flight while they were small, arboreal animals. Our ancestor was indeed small and arboreal as tree shrews are some of our closest relatives. The lorises and tarsiers are also our close relatives, and they are small and arboreal. Despite being small and arboreal, they did not evolve gliding or flight. It is a road that is not taken. Some animals choose to try to fly, but others do not and are content to climb among branches on the forest canopy instead. Different strokes for different folks.

A bird can fly because its wingspan and the wing muscle strength are in balance with its body size. It has a lightweight skeleton with hollow bones, which puts a smaller load on its wings. A bird also has air sacs connected to its lungs, which makes it even lighter and allows for easy passage of air through its lungs during flight. On the other hand, calculations of the ratio between human size and strength reveal that our species will never be able to take flight unaided. As an organism grows, its weight increases at a faster rate than its strength. Thus, an average adult male human would need a wingspan of at least 6.7 meters (22 feet) to fly. This calculation does not even take into account that these wings themselves would be too heavy to function.

We'd need a wingspan like a hangglider, and have nowhere near the muscle-power to flap wings that big.

Apparently, no creature over 40 kg flies on its own power

There is a reason that there are no flying birds that weigh as much as adult humans. (There are some that heavy but they cannot fly.)

The reason is because of the "square cube law". As you increase the size of something linearly, the surface area increases by the square. This would greatly increase the lift that is used in flying. But the volume (and weight) increases by the cube! This far overshadows any gain by having a larger surface area.

Humans are much too heavy to fly with muscle power alone.

Wow, that's a tough one. Let's see, could it be that humans don't have wings? Nah, that's too obvious.

Alas, we do not have wings.

We weigh to much we would need very large wings just to get enough lift