Nature is fairly economical with energy. Large birds, for example, sail over long distances with minimal energy investment. Other animals shut down or throttle non-essential functions, or simply do nothing most of the time in order to minimising the amount of food and water needed to keep the system going.
Humans have evolved way beyond the point of reason, so much that we actually need to exercise to burn off the excess calories.
I don’t think excess calories are on a fly’s mind, though.
Why the heck can a normal fly not just sit still on the window and admire the sight? Why do they have to buzz around all the time like mad? Given that they don’t feed that way (I presume they mostly feed on horse manure and the content of my compost bowl), what’s the point of that constant buzzing around?