Nerve endings etc to send the information we see to the brain; The macula, which is a tissue that helps us process color vision as well as see things with clarity iirc.
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The main body of the eye (vitreous body) also helps with shock absorption from minor impacts, and it's shape allows the retina to remain attached, as well as to give the eye more movement rather than being in a fixed position, looking straight ahead type deal
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Iirc (and someone should correct me if this is wrong) the eye is also fairly dependant on moisture to maintain function. A dry eye will have cells start dying, so it works better to only expose the parts of the eye absolutely necessary to see and keep the rest safely encased
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Very basic

Real eyes realize real lies

*very basic explanation of some of the reasons our eyes are kept mostly behind enclosed doors

cuz we can