Upvote:3
The simple answer is that some people so interpret the Bible as to turn literal aspects of such things into symbols for something else that is non-literal.
You further ask how such persons are justified in their stance. Well, those who take this stance could answer at length, many of them using parts of the Bible to support their interpretations. To their way of thinking, they are perfectly justified.
It's not surprising that many Christians cannot understand why some people do not believe in devils and hell (as a place of eternal torment, I assume), but the only way to learn the answers is just to put your questions out there and allow responses. Your invitation for them to explain themselves may be taken up by some, but as I am not one of them, I cannot give a detailed answer.
Yet having come across many such people in my lifetime, I can add that such differences in views is down to how the Bible is interpreted, if, indeed, the Bible is even regarded as a sound basis for interpreting such beliefs. Many people who say there are no demons and no hell as a place of eternal torment also say the Bible is a motley collection of ancient stories and unreliable reports, mangled over the centuries because it was only fallible men writing and copying them. Once such a stance is taken, it's very easy to play fast and loose with the contents of the Bible, and with what they say it really teaches.
I'm just giving a basic answer because of the criteria for Stack Christianity; 'truth' questions are not 'allowed', nor are ones that seek opinions, or ones that will gender arguments. But if you really want to know detailed reasons, some might detail them.