Most of that inexpensive amo is just like looking in a cracker-jack-box.

You don't know what you got in there.

You never know how many grains of power is in each case. The box might say 85 grains, but it could easily have 87 or 82 grains, in which case your bullets all go in different places.

Reloading is less expensive, and you know what you are shooting. The bullets end up the same place each time.