A couple points:
1. ISIS is religiously motivated to destroy the influence of the US. Check out this documentary here: https://news.vice.com/video/the-islamic-state-part-1. This means that rationality, though a factor, is further down the list and makes them dangerous. They don’t equate dying with losing.
2. they know they don’t have to destroy the US (or the West) through an atomic bomb per se, but rather destroy by fear (terrorism). What that means is that they can perform a high-value strike (symbolic) that takes a major psychological toll on a fragmented, financially and morally weakening people. The appetite for suffering here in the US is at an all-time low – people have grown accustomed to having it as smooth as possible and have little tolerance for discomfort. This equates to an easy target.