Because they have been brainwashed form childhood to believe that their version of 'god' is the only true version - everyone else's version is evil.
Ask yourself this question - if Israel was, indeed, promised to the Israelites by god why did they have to slaughter thousands of men, women and children (and - in some cases - even cattle) to take possession of it? Israel - according to the Bible - was founded on carnage.
The Bible isn't - by and large - a historical document. There are, however, a few passages in it that allude to actual historical events - all of these allusions relate to battles and warfare. Much of what is left alludes to mythical battles and wars. THE BIBLE IS ALL ABOUT WAR.
Getting people to fight each other for a cause (the further enrichment of those already rich) is the PRIMARY PURPOSE OF RELIGION.