Because throughout pretty much all of human evolution, and up until relatively very recently, reacting quickly to things in your peripheral vision was very important for survival. As part of that, it’s safer to assume something is a threat (and be startled) than to assume it’s friendly and potentially lose that precious reaction time.
Basically, even though you know you are in a safe place, your subconscious mind still has hundreds of thousands of years of instincts tell you that any movement in your peripherals is a potential threat and to act accordingly.
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