Don't you think it's kind of arrogant of man to think nothing can exist that is outside his perception?
I don't think anybody thinks that.
What we think is that, if it's outside our perception, it can't affect anything
in our perception, because then we'd be able to percieve it, at least indirectly.
Anything "supernatural" - disconnected from the natural universe - is by definition undetectable and unable to influence events in the universe. If it
could exert influence, it wouldn't be supernatural, by definition.
If it can't ever affect us, and we can't ever detect it, who cares if it's there or not? Why bother thinking about it when there's literally no way to determine if your thoughts are accurate or not?