Some people (like me) take it seriously because we do not see any evidence to support much of what modern geological or biological scentists suggest.
Like scientists, we begin with a bais. The scientific bias, which is understandable, is that science must only consider the natural and natural explanations for what can be observed. People of faith often begin with the bias that the supernatural must be considered.
If there is a God who is truly omnipotent, then it is completely possible - though not naturally provable - that he created all things in six days and rested on the seventh. If there is no God, and no supernatural forces at work in creation, than modern cosmological/geological thought and evolutionary biology is the most likely explanation for how the world got to be it is today and how we came to exist.
This is not to say that there are not those out there who are able to reconcile evolution with religion. There are, and that is their perogative.
I think for many Creationists, the major problem they have is that they feel the continued battering of society with a theory that is explicitly unable to credit the supernatural with anything serves to undermine the veracity of their beliefs, and poses a significant issues when it comes to forcing children to learn it as a condition of being educated. It is, thus, viewed as one of several fronts on the culture war that most Creationists (Christian Creationists, anyway) feel is going on between society at large and the Church.