Because a lateral miss does not prove that the rod tip tracked in a curve. An unsuitable amount of delivery energy can make the line, leader and fly kick left or right at the end of the cast, and a sidearm delivery shows that kick clearly: the loop is already travelling close to the horizontal, so a sideways finish arrives as a lateral miss.

Turnover is the line, leader and fly unrolling and straightening out at the end of the delivery. The delivery energy has to suit the fly and the cast. When it does not, the end of the line kicks sideways instead of running straight out, the fly lands off to one side, and the caster blames the aim.

So check turnover before you decide the tracking is curved. Watch the fly leg: does it unroll straight, or does it swing out to one side at the finish? Then look at the rod tip path and the aim. On a sidearm cast the two faults can look almost the same from behind the rod.

Investigate turnover first because it is the cheaper explanation to test. A tracking correction applied to a turnover problem will not fix the miss.

Source

From Sidearm accuracy and turnover kick; see also trajectory and accuracy and the definition of tracking; terms follow the FFI Fly Casting Definitions.