We can measure energy losses though. If you're brave and/or confident in your shooting abilities, you can put your chrono out at set distances and measure the speed.

The arrow weight is fixed- it's the same coming out of the bow as it is at 80yds. You can measure the initial velocity of the arrow coming out of the bow. That will tell you your initial KE value.

Then you can measure the speed of the arrow at the target at different distances. Say I put my chrono in front of the target at 60yds (being the brave person I am). I measure that speed out at 60yds. The weight is the same so now we know the KE value at 60yds.

Subtract the value at 60yds from the initial value, and that will be how much energy is lost from drag, vibration, inefficiency of the arrow to bow combo, etc.....

But you're right- you can't single out each loss factor and determine how much energy is being lost for each parameter by itself. (this amount for drag, this amount for arrow flex, this amount for improper flight)