I used to ask the same question...but, one scripture keeps playing over and over again: "There is a time to love and a time to hate; a time for peace and a time for war." And then you have: "for we do not wrestle against flesh and blood; but against PRINCIPALITIES and powers; spiritual hosts of wickedness in heavenly places." (verbatim) What we are "fighting" are PRINCIPLES: what people believe and stand for. Some believe cash rules the world...some believe it's goodness and kindness. Positive and negative forces are constantly waging war on each other. That IS the battle and Earth is a battlefield.

Here's another idea:

Perhaps wars and conflict are a necessary "balancing" of the duality of existence. Since there can't be a light without the dark, maybe there can't be peace without conflict either? Perhaps both must exist, or take turns in cycles, like the Ying and Yang union of opposites in Chinese Taoism? If so, then it would not be realistic to expect or hope for a world full of peace, joy and goodness right?