I thought the same thing when I went and bought AF. Then, after adding to the system, I wished I had bought pre-mix. If you add AF then water you can easily get the ratio wrong. That leaves you with mixing before you add to the system. In reality, we don't use that much so the cost difference is minimal and the hassle factor is much lower.
This might be helpful for some. I am estimating these off a graph so the temperatures are approximate. % Glycol, Freeze Point(F) 0 , 32 20 , 17 40 , -10 50 , -35 60 , -70 70 , -65 80 , -50 90 , -25 100 , 10 Sorry the two columns got squished and I don't know how to fix that.