The only reason that ISIS is expanding and doing so well militarily with lots of American equipment is because Obama doesn’t want to stop them and is conducting an ineffective air campaign for PR purposes only.
Raqqa is isis’ capital, and has not been bombed. What does that tell you?
You greatly underestimate them, and the situation that gives rise to them.
Militarily, I agree they represent a minor challenge, and their forces could be destroyed fairly quickly by an MEU with appropriate air and logistical support.
But their success is not principally military. The people of Northern Arabia have been denied their preferred means of social organization for almost 100 years (or, more exactly, the means of social organization they were deprived of in 1919-1921 and which loss they have been taught explains their shame and their poverty).
After ISIS is removed by force (if that were to happen), the problem of how to govern a primitive muslim population that wants a caliph will remain.