That is their stated goal, but their actions show otherwise.
They mostly fight other Sunni Muslim organizations like Taliban, Hamas, Al Qaeda, which does not help in their stated goal.
They claim to follow the teachings of an author called Sayed Qutb, who is a really big figure in the Muslim brotherhood (which Hamas comes from) yet they attack Hamas.
Also
IS ideologues rarely uphold adherence to Islamic scholarship and law manuals for reference, mostly preferring to derive rulings based on self-interpretation of the Qur’an and Muslim traditions
Compare ISIS with the Taliban, an actual fascist organization which wants to rule in such a manner. Taliban mainly operated in Afghanistan, against invaders be they Russians, NATO, whatever. They did not operate in foreign countries like ISIS does. ISIS openly admits that it wants to destabilize other countries and radicalize Muslims living abroad to convince them to commit acts of terror
ISIS wants to stir religious hatred in Europe and the United States – so that Muslims no longer feel they belong in the West, and either carry out attacks in their homelands or leave to join the caliphate.
If they genuinely wanted to establish a caliphate they would first focus on gaining regional dominance, then either freeing Al quds, and occupying mecca.
Instead they fight Hamas, and try to get Muslims to commit acts of terror abroad