Ok, lets take a logical step back from whats been said so far.
Belts wear, period. But this process can be increased to a few things.
1) Binding transmission. With a binding transmission it can stretch the belts with the increased loading put on them.
2) Foreign objects wearing the belt out. This could be internal to the car, such as a loose screw/bolt etc and fraying/scrubbing the belt, or external (such as a stone/whatever) getting into the drivetrain and rubbing the belt up.
3) Worn bearings. With worn bearings, the belt may have more load placed on it every time the motor is spooled up, as there is more movement in the whole belt/drivetrain setup. Replace the bearings and the loading to the belt will then be smoother on spool up.
4) belt tension, already covered, but one which isnt set at the right tension will a) wear prematurely and b) possibly put an increased load on the surrounding drivetrain bearings.
5) To a certain degree, the motor and chosen gearing may play a part into how quickly the belts will wear, but not as quickly as you are describing.