It is because they are gullible, insecure and easily deceived. The expansionist empire-building of any nation is only ever an inverse function of the doubt and fragile uncertainty upon which they have been built. The proof of this is not difficult to determine – notice that every adversarial and belligerent gesture only ever serves to self-validate and reproduce the insecurity and experience of fear that obligates the need for aggression and violence. You will see this within individuals, between people or tribal groups and within or between nations.
In the end, this is all and really only ever that self-propagating dissonance of complexity that travels as a wave of compression through all if us and towards which we are so often reflexively oriented, not because we benefit but rather because the patternsvof difference as information themselves optimally self-replicate through and as turbulent vectors of belief and intractable difference. In this sense, the wars are not even ours and any benefit anyone might ever gain is entirely transient and inconsequential. It is not humanity that benefits from wars, although any compassionate soul need hardly be reminded of this; it is the harmonic resonance of a patterned complexity that takes us all along for the ride.