They are fighting because they believe for the first time since the Ottoman Empire fell, they can win the war. That was the lesson learned in Afghanistan in the 1980's.
Islam has suffered centuries of retreat and defeat. Their territory has decreased and they were no longer following Islam's mandate to "take over the world."
So, starting with the Egyptian Muslim Brotherhood (and a little before that), they saw that the fault lied with them. If Islam was on the retreat, it was only because Islam had been compromised and Muslims were living in apostasy.
They returned to a simpler form of Islam. Returned "back to the book" so to speak. (Just like when Christians say we have to return to the Bible to solve our ills)...and it was a return "back to the book" that caused this explosion in Jihad that started during the Mandate period.
So what we are seeing was not started as a desperate fight for survival, but as a religious revival (let me use Christian terminology again)... and it is that revival that is the root cause of what we are witnessing today.
A good observation. In fact, it's both.
As I mentioned earlier, al-Qa'ida is a political organization masquerading as a religious one. Islamic scholarship and philosophy is important to al-Qa'ida, but ultimately, they're about changing the here and now, not debating points of theology.
Islam is not merely a religion or a form of government. It is, strictly speaking, a cult(ure) that manages both the spiritual and the temporal. The rise of Western military and economic power over the last several hundred years was unfortunate, but it never threatened Islam culturally. Islam is simply too insular to be affected by such things. Western power simply meant that the non-Islamic barbarians were too powerful to for the Arabs to continue enlightening them. Even when Islamic lands were conquered, the colonization efforts never took more than a superficial root.
That changed over the last century or so.
The Middle East had fallen so far behind that even the inadvertent changes they absorbed were starting to change their culture. Mass media showed the Arabs how backwards they really were. Easy travel and telecommunications brought the outside world to their doorstep. Military defeats, not by massive Western armies, but by a handful of Jews (who were then seen by Arabs not as powerful and menacing, but as the butt of jokes), paraded their weakness in front of the world.
In short, Islam's claim to be the end all be all of human advancement was shown to be wrong on all counts. Western ideas penetrated the region. Syrian socialism, Iraqi secularism, Egyptian nationalism, Bahrain's Las Vegas like atmosphere, everywhere was seeking aspects of Western culture they liked, and trying to install them instead of Islam. On an individual level, Iranian girls own miniskirts, guys listen to rock music, and everyone parties in private. Sure, the people were Muslim, and believed in Allah, but Islam as the dominant guiding principal of daily Arab life was being discarded on every level. Deep fault lines emerged in what was once a monolithic entity.
Now, you can't just toss aside a cult like that and not expect consequences. It is, at it's core, a lifestyle mandated by Allah. Hardliners realized the threat that this corruption posed, and have launched the revivals you're talking about. No doubt about it, the retrograde, 8th century idealistic Islam is making a comeback.
The comeback is at hand because they're mobilizing in the face of a threat. Islam is always in a state of conflict when it borders a non-Islamic nation. Still, what's going on now is almost a frenzied response, like someone who is flailing because a garrote was slipped over their neck. Western ideas and technology threaten to suffocate traditional Islam in a way that wasn't possible before, and that has given rise to the militant, reactionary response we see now.