I found the answer myself. That's because the background image has "data-speed: 0.1". #wrapper has a "position: fixed;" value, so it works well when I put a background image outside #wrapper.
I don't know, but I think if you put the value "data-speed: 0.1", the "position: fixed;" attribute will also be automatically added. I don't know how to erase this topic, so I'll just leave it.
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