There are several reasons for this. One is that in time Nylon will stretch and deteriorate. I do not agree with the severityof the rule as written. However, I do believe that after any major inpact the belts need to be replaced. The failure of the Earnhart belts has been discussed here and in many other places. The current issue of Speedway Illustrated has an excellent piece showing installs that are wrong or that cause fraying due to entry/exit. I have wondered about this issue too. My car is heated garage kept under tarp. It is on track 3 weekends a year-there is effectively NO wear. Is 3 years unreasonable-NO. But, then again some folks are buying very expensive 5 year rated belts made of the SAME material as the 2 year ones How is the price justified--by the FIA tag and nothing else as I see it.
Just my humble view.