Why believe in something that is so fictional
I'm not sure that question can be answered directly.
People who believe in God don't think it's fictional, so they won't be able to answer. And people who think God is fictional won't be able to answer directly because they not believers.
Since I'm not a believer, all I can really do is guess, but I'm sure there are lots of reasons for people to believe what they want, not the least of which is "brainwashing" from their youth (because I suspect that a VERY high percentage of believers, just happen to believe in the God they were exposed to by their family and friends, even though there are thousands of god concepts that they could have chosen).