How did the war last so long?

decepticons 's anstiors start this war with autobots 's anstiors

I'm not sure if this would apply to the movieverse, but in the IDW comics they recently addressed this (which I'm glad because I always wanted an answer to this as well).

I think it's in issue 24# of the ongoing just previous to the split into the two new titles MtMtE and RID. Megatron and OPtimus are reminiscing on how long they'd been at each other's throat and in that conversation, it's mentioned how each side would spend centuries calculating battle plans, gathering offensives, soldiers, weapons, strategems, etc only to finally go at each other and wipe each other out in a few days, only to turn around and begin to build an army all over again. And this is for strategic points across the galaxy, not just key points on Cybertron. So they're not always in "active" war, spending a lot of time in the building/planning stages. Like chess.

Which makes sense because these are beings with a lifepan of hundreds and thousands of years. Combined with the fact that they can think with the cold and pinpoint calculation of the most powerful computers in the universe as the norm, and it's easy to see how long the war could last so long.