"Why is it reasonable to believe in prisons, but not in the hell?"

[FlatAssembler]

Has anyone seen the Biblical hell?  No.  Therefore there is no proof hell exists. 

Has anyone seen the inside of your skull? No. Therefore there is no proof your brain exists.

But people have seen other brains. And it is a scientifically accepted fact that any living person has at least a part of a brain in his skull, because a human being cannot (unlike insects) live without a brain (one cannot breathe without a brain, for example). Seriously claiming a living human being has no brain is a very extraordinary claim, because it requires much of what we know about biology to be false.

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And hell not being justice is also not a valid argument against the existence of hell?

Correct.  The injustice of hell as a concept or a real place says nothing about it's existence.

Do you understand?

Well, maybe. Still, many atheists use the argument that hell would not be justice as an argument against its existence, and I think it is a relatively convincing argument. I used it in my latest YouTube video about atheism.