Why do people believe that Beowulf is fiction?

[Angrboda]

Not the flying fire breathing dragons of myth you haven't.

At least not in genuine science books. I've heard that the discovery channel's standards have dropped recently but I don't actually watch it.



Parrots don't understand what they are saying. They are learning to repeat patterns of sound that they are trained to repeat.

Gorillas can be taught limited sign language by humans but that's about it.

You are correct, I haven't seen flying fire breathing Dragons.   And I would agree, that Parrots cannot carry on much of a conversation, although they can learn to repeat simple things to communicate.  Similarly, Gorillas capability to communicate is limited from what I've seen.   However, I question the simplicity of such rules; given previously such as these.   They may be fine for children, but for thinking adults, the reasoning and principles I believe are sketchy (or perhaps it is just a left over from children's rules)    Things such as do not cross the street, or do not talk to strangers. 
While they may not be called dragons, there are some amazing and similar things within nature.  Animals that spit or expel an acid at attackers.   Bombardier Beetles mix chemicals to produce a near boiling spray as a defense.   Mimic Octopus do some amazing things, changing color, changing shape, and imitating other animals with uncanny accuracy.  It seems difficult to dismiss by any principle or reason a story, because it contains a fire breathing creature.   It appears more like saying that "I don't believe in fire breathing dragons, so I don't believe the story"  but that is not saying much, if it is more based on a rule for children; rather than reason and principals of thinking adults. 
Another problem with such simple rules, is that there can be misunderstandings.   Such as my previously mentioning of dragons, some might imply fire breathing and flying lizards/serpents  when that is not what I intended at all.  Or they might have something else in mind when speaking of talking animal, than I do.  Given this, I think that we should allow for the principle of Charity when evaluating a story.   Not to be so quick to call someone a liar or to claim that something is fiction based on what could be a misunderstanding on our part.
Lastly one might cite experience as a reason to dismiss such things.  However I would question that as well.  I would make the distinction here, between contradicting our experience, and something which does not fit with one's personal experience.  In the first instance, if it is necessary to choose, between two opposing, then the one which is better supported should be kept (although we do have to examine and not just dismiss out of hand to know that it is better supported).  The latter just means that we may have to accept something new, perhaps change our view, but don't have to get rid of well established experience.   Because something doesn't fit within our worldview or experience, it doesn't follow that it is a work of fiction or a lie.  I don't think that we can learn, and grow, I think that science is weak, if we stick to the rules of children, rather than following evidence and reason.  Too many times, I've heard scholars and laymen alike declare that something doesn't exist or something cannot be done, because it falls outside of our current subjective experience, and later they have to eat crow.