They've been created by the US for this exact purpose. They love having a bunch of terrorists they can control. This way the US government sits on the sidelines telling them what to do while the terrorists do all the dirty work.

• US spy got caught in turkey recruiting for ISIS: http://www.takvim.com.tr/guncel/2015/07/05/altindan-abd-cikti

• German Journalist Ken Jebsen: USA created ISIS | Eng Subs https://www.youtube.com/watch?v=PFB4c0OVEtM

• Now the truth emerges: how the US fuelled the rise of Isis in Syria and Iraq http://www.theguardian.com/commentisfree/2015/jun/03/us-isis-syria-iraq?CMP=share_btn_fb

• Pentagon report predicted West’s support for Islamist rebels would create ISIS Anti-ISIS coalition knowingly sponsored violent extremists to ‘isolate’ Assad, rollback ‘Shia expansion’ https://medium.com/insurge-intelligence/secret-pentagon-report-reveals-west-saw-isis-as-strategic-asset-b99ad7a29092

• Newly-Declassified U.S. Government Documents: The West Supported the Creation of ISIS http://www.washingtonsblog.com/2015/05/newly-declassified-u-s-government-documents-the-west-supported-the-creation-of-isis.html

• FBI Breaks Up Another Of Its Own 'ISIS' Plots, Where It Supplied Most Of The Planning https://www.techdirt.com/articles/20150410/11564430610/fbi-breaks-up-another-own-isis-plots-where-it-supplied-most-planning.shtml

• US Sending weapons to ISIS http://www.globalresearch.ca/delivery-of-us-weapons-and-ammunition-to-isis-iraqi-commander-wiretaps-isis-communications-with-us-military/5437627

US/British strategy in the Middle East has been 'divide and contain' since the partitioning of the Ottoman Empire after WW I.