Because the USA government funded ISIS to begin with? There is no advantage to removing ISIS they aren't a threat to the US but are anti-Assad so they support our interests in the region indirectly.
Furthermore they are great scapegoat for an executive government power grab and building a surveillance state.
It doesn't hurt that prolonged military engagements do wonders for the profits of the military-industrial complex either.
I'm ALL for taking out ISIS. But, with carpet bombing huge areas, the likelihood of unintended collateral damage is very high. Which brings us to the general attitude of the U.S. in war - "When it comes to innocent lives, only U.S. lives matter!!"
There's ZERO chance that the U.S. would tolerate collateral damage on it's own citizens in the name of getting ISIS, so why accept the possibility of non-American causalities?
Furthermore they are great scapegoat for an executive government power grab and building a surveillance state.
It doesn't hurt that prolonged military engagements do wonders for the profits of the military-industrial complex either.