Why must islamic terrorism always be compared to some fringe group or phenomena?
Because, relatively speaking, it is a fringe group or phenomena.
There's a really fundamental misunderstanding here of what Islam and Islamism actually are.
When I was a kid, noone knew or cared about Islamic terrorism. The mujahideen in Afghanistan were those cool guys who helped Rambo fight the Soviets in Rambo III (which was itself propaganda covering up the fact that western intelligence agencies armed the mujahideen in Afghanistan, just like they armed Saddam Hussain). If you had to name a terrorist organisation, you'd probably name the IRA, but noone knew what the hell an Al Qaeda was.
This supposedly timeless and inherent relationship between Islam and terrorist violence is less than two decades old (although the ideological roots go back to around the late 19th century).
Islam is a legalistic religion, like Judaism. The traditional sources of authority in Islam are legal scholars, theologians and philosophers (who collectively form the Ulema). The theoretical head of the Islamic religion is the Caliph, but there are only four Caliphs in history whose authority is recognized by (nearly) all Muslims. For the vast majority of Islamic history the authority of the caliphate has been nominal, often with multiple claimants. For this reason, there is no single interpretation of Islam that is deemed universally valid, and for this reason "fundamentalism" is an alien concept to traditional Islam. That's why the Ulema are so important, because the act of interpreting and forming legal judgements based on Islamic tradition is a complex task requiring expertise.
Ijtihad, the excercise of individual reason to interpret religious texts, is an important part of Islamic tradition.
All of the Sunni terrorist organizations in the world today are wahhabists. Wahhabism is a general term for an extremely puritanical reformist Islam. Wahhabism arose independently in many countries, largely inspired by contact with the West and in particular with reformed Christianity). Wahhabists view the Islamic tradition as corrupted by superstition and spiritual weakness, and seek to return to what they see as an original "pure" Islam expressed through the Quran (and to a
much lesser extent the hadith). To this end, they reject most sources of traditional Islamic authority unless it aligns with their fundamentalist reading of these sources. They believe that the adoption of this purified Islam will lead to a spiritual rejuvenation of Islamic society, possibly including the restoration of the Caliphate, the end of Western dominance over the Islamic world, and the destruction of schismatic traditions.
We tend to think of fundamentalism as ancient and reactionary, but in reality fundamentalism (regardless of religious tradition) is a highly rationalized form of religion that only appears in the modern world. Today, most religious traditions have some form of fundamentalism, but those fundamentalist traditions only came about quite recently.
The fact that there are no truly secular democracies in the Islamic world is not due to the influence of Islamism, it's primarily due to the Western political hegemony over the middle east. Western intelligence agencies have largely supported reliable autocratic governments rather than unpredictable democratic governments, because autocratic governments can be trusted to act in Western interests. Secular nationalist dictators like Saddam Hussein were, at one point, seen as reliable Western allies and supported and armed by Western governments. Many middle eastern governments also have a problem with rentierism, which is when a large proportion of the state's revenue is derived from foreign companies and interests (usually the fossil fuel industry and Western governments) rather than from taxation of its own citizens. This means the state is essentially not required to be accountable to its citizens, which tends to result in autocracy.
There is no great appetite for liberalism in the Islamic world, because it's incredibly naïve for anyone in the Islamic world to believe that a liberal government will actually protect their interests over those of its Western backers. Islamism, on the other hand, has a proven track record of standing up for the interests of Muslims over those of Western governments. The primary driver of the current surge in global Islamism isn't
the inherent violence of Islam, it's the geopolitical situation, and in particular the ongoing perception (which is largely accurate) of Western interference in the Islamic world.
The fuel you put in your car has to come from somewhere.