Well, the towns get attacked by sin every once in a while and chances are that some of the statues are destroyed either by sin (or sin spawn... remember the one that got into Kilika temple) or for some other reasons (seeing how corrupt yevon is).

And just to prove that there were quite a few, did you notice how many statues surounded Remiem temple? My brother [Rikku_007] thinks there must be over 100 of them, and the temple stopped getting statues quite a while before the start of the game (its use discontinued).