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Because your mind says its rectangular.

If you actually widen your eyes and look out of your periphery vision (on both sides at once) you will notice that wall curves down/up towards a vanishing point.

If you take photographs and put them in PS™ and then draw straight lines, you will notice that everything you thought was a straight line is actually curved. (except the horizon)

I'm not sure that has anything at all to do with objects that don't fill my entire field of vision (like the Sun and the Moon).

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