Because, you cannot...tune a gun without a chronograph !
Without knowing what the pellet speeds actually are, you have no idea where to go from your starting point, or anywhere else.
I have a Magnetospeed V3.
I like it because I don't need a bench/table to set it up on. The Magnetospeed attaches to the barrel, similar to the FX. While the FX is a little easier to setup, the Magnetospeed has (so far) none...of the FX's battery eating problems that many talk about. I've tuned all (a bunch !) of my adjustable guns, some more than once, all on the same batteries. It seems pretty repeatable also.
Mike