They're not. Some of them get that way when they are bombed, their nations are invaded, and they live under dictators which the United States supports.
That's the gist of this thoughtful article by Glenn Greenwald, excerpted here:
First, some leading American opinion-makers love to delude themselves and mislead others into believing that the US is attacked despite the fact that it is peaceful, peace-loving, freedom-giving and innocent. As these myth-makers would have it, we don't bother anyone; we just mind our own business (except when we're helping and liberating everyone), so why would anyone possibly want to attack us?
With that deceitful premise in place, so many Americans, westerners, Christians and Jews love to run around insisting that the only real cause for Muslim attacks on the US is that the attackers have this primitive, brutal, savage, uncivilized religion (Islam) that makes them do it. Yesterday, Andrew Sullivan favorably cited Sam Harris as saying that "Islamic doctrines … still present huge problems for the emergence of a global civil society" and then himself added: "All religions contain elements of this kind of fanaticism. But Islam's fanatical side – from the Taliban to the Tsarnaevs – is more murderous than most."
These same people often love to accuse Muslims of being tribal without realizing the irony that what they are saying - Our Side is Superior and They are Inferior - is the ultimate expression of rank tribalism. They also don't seem ever to acknowledge the irony of Americans and westerners of all people accusing others of being uniquely prone to violence, militarism and aggression (Juan Cole yesterday, using indisputable statistics, utterly destroyed the claim that Muslims are uniquely violent, including by noting the massive body count piled up by predominantly Christian nations and the fact that "murder rates in most of the Muslim world are very low compared to the United States").
As the attackers themselves make as clear as they can, it's not religious fanaticism but rather political grievance that motivates these attacks. Religious conviction may make them more willing to fight (as it does formany in the west), but the motive is anger over what is being done by the US and its allies to Muslims. Those who claim otherwise are essentially saying: gosh, these Muslims sure do have this strange, primitive, inscrutable religion whereby they seem to get angry when they're invaded, occupied, bombed, killed, and have dictators externally imposed on them. It's vital to understand this causal relationship simply in order to prevent patent, tribalistic, self-glorifying falsehoods from taking hold.
Second, it's crucial to understand this causation because it's often asked "what can we do to stop Terrorism?" The answer is right in front of our faces: we could stop embracing the polices in that part of the world which fuel anti-American hatred and trigger the desire for vengeance and return violence.
Also, good reading: Juan Cole.