Because no one ever bothers to clean, let alone lube all the linkages under their mower & / or deck , holes flog out oval.
Then the actual pulleys wear so the belts sit deeper into the groove .
Thus the "correct" size no longer fits.
This is before all of the "fits a abcd" belts are taken into account where the belt angles are standard where as the OEM belts are not thus again either the belt wears out in no time flat or breaks.
Nothing odd to fit a beck belt better than 2" shorted to keep an old mower running.
Not so bad with the electric clutch models but the moving deck PTO's can be a real pain, particularly MTD.
Cub do not and never have used std section belts they are wider and a lot of Murray belts are deeper.
OTOH a $ 75 belt is a lot better than $ 900 in pulleys & linkages, including a $ 50 belt.
Belts are like most things high volume = lower prices and I don't know how many totally knackered Vari- Drives I have resurected because the owners were too cheap to buy genuine MTD belts and ran std belts which usually twist then invert and cut deep grooves into the pulley so in place of a gradual inclease in speeds it jumps strait fro 2nd to 6th