Because people believe in it. It may not be though, you can believe whatever you want to.
Cause god tell us
And if u believe in god u will know
scientifically, nobody knows. but some people choose to believe in it despite this! i believe in heaven and hell because i believe in christianity and that's just another part of it. some people believe it for different reasons, like that they're hoping they won't have to die, or it's reassuring after the death of a loved one. people believe that it's somewhere in another place, that the living can never find, which does mean that we'll never really know unless it is there when we die. it's a belief, not a fact.