And perhaps that is the point. Stupid, ineffective, low-cost attacks which rile Western nations and put pressure on greater forces for intervention, leading to a land invasion of Islamic State territory, which can then be used to rally Muslims against the "Western Crusaders and their Zionist masters".
I'm betting it IS the point. ISIS doesn't exactly have the crowned heads of the Middle East on board. They aren't going to form a monster Islamic state without a common enemy, at least to shake things up a bit.