Due to the following reasons:
1) Most of the generators (small) are generally with single cylinder engines where as auto engines are 3, 4, 6 cylinder engines. Multi cylinder engines maintain balance ( vibration) while rev. at various speeds as compared to single cylinder engines.
2)Generators have a fixed rpm (to maintain frequeny Hz) irrespective of the load, where as vehicle operate over a wide range of rpm as per load requirement. Operating a Diesel engine generator at less load ( less than 40% or so) reduces the engine life.
3) Operation of vehicle engine measured in terms of distance travelled by it; where as the generator engine operation is measured in terms of running hours (time).
4) Generators are some times run with loads exceeding 110% for a long period of time than recommended without acknowledging the wear and tear and stress on the engine parts thus reducing the engine life.
5) Generator cooling system is the most neglected system and is only attended after a break down has taken place.
And so on.