At the end of the day its cheap insurance repacing them isn't it? It's also basic engineering/science principles.
Have a read up on torque and yield. In basic, when you torque the bolt up you use up its 'elastic' range, at the very least a cetain % of its max tolerance. Re-using the bolt will require a re-torque, using a wrench thats most likely not calibrated and quite possible to go beyond its limit. Add to that fatigue due to age, heat, and chemicals, and you have a bolt thats structurally 'past it'
I'm sure many have re-used and had no problems, but i guess thats the risk you take.