Kilimanjaro, the toothed timing belt has severe intermittent loads from the cams and has to maintain very precise synchronisation between the cams, the crank and the diesel injection pump, and they have a definite wear limit. The change interval (60,000 miles or five years) is dictated by worst-case wear, so many belts have a lot of life in when changed out. However the consequences of a failed belt are severe (bent valves at the very least, piston damage or even bent conrods at worst) so it makes sense to change them at a relatively low interval.
A chain would be better, and would not need changing, but would be noisier.