Usually terrorists trying to take over a state take over an already established one. It's easier that way, in terms of international recognition, the existing machinery of state etc. See the FLN in Algeria for possibly the ur-example of this type of thing (then watch Algiers).
ISIS isn't looking too hot right now. Seems like everyone has decided to put them at the top of their shitlist, and their primary strength has been playing the sides against each other while it did its own thing. Now the Iraqi Army (+ Shiite death squads) are attacking from one side, the Kurds and SDF are taking huge swathes of territory in the north and the Syrian Arab Army is attacking in the south.
They'd also probably be getting attacked from the west, only there's a desert and a bunch of mountains there, effectively pinning them in place. So they're kinda fucked.
More broadly speaking it is very hard to get recognition for a breakway region without great power (and specifically American) backing. Russia has been supporting breakway statelets with far more claim to nationhood than IS for decades, and no-one except Russia and Belarus recognise those poor sods. By and large America supports the international status quo, and the international status quo is to accept existing boundaries and states where possible. Compared with the previous 150 years, the post-Cold War period has been remarkably stable in this particular way, and I suspect American strategists are afraid what could happen to that stability if that norm were undermined.
And quite frankly, no-one wants anything to do with ISIS. Even Turkey keeps their support covert, because they find ISIS repugnant on a very basic level. No-one is going to waste their political capital on putting forward the case for Islamic State at the UN. No-one wants to give Islamic State the right to build embassies, or treat their insurgents as professional soldiers.
Sooner or later we will have to talk with ISIS, because the alternative is a neverending bloodbath. But I suspect most of the world prefers that conversation be done at gunpoint, and involves convincing moderate elements to stand down and accept incarceration for war crimes.