"...it was as if someone just had to create the Islamic State as an excuse to reinvigorate the need for more military interventions."

There are a number of reasons why, despite the early evidence of genocide by Islamic State, the USA allowed it to grow and take over vast swaths of territory in Iraq and Syria.

1. Saudi Arabia is the primary ally of Islamic State, and the USA is Saudi Arabia's best friend.

2. Islamic State is a thorn in the side of the Shi-ite controlled government of Iraq, and its ally Iran.

3. Islamic State's greatest territorial gains were against the government of Syria, which the USA and its allies are trying to overthrow.

Islamic State was (and still is) a very useful tool for the USA. That's why it wasn't squashed in the beginning, and its why it hasn't been squashed since.