A piece of fiction is a carefully-crafted series of lies told to the reader, which the reader agrees to believe for the duration of the story. Anything that threatens the fragile nature of this agreement is bad.
This is, for example, why it’s necessary to do your research – if you get something wrong that your reader is in a position to notice, the reader’s ability and desire to continue playing along with you is going to be compromised. The same goes for poorly-motivated characters and plotting by incredible coincidence.
It’s just one of the many oddities of the writer’s craft. We are, or at any rate ought to be, scrupulously honest and careful about the small stuff, all in service of a bald-faced falsehood:
Listen. I’m about to tell you a true story….
JAMES D. MACDONALD 