The up–down orientation unquestionably comes from the near-universality of light sources being above you. You'll notice that intense up-lighting is almost never found in the natural or artificial world.
(It's true that upwards-firing lights are often used in interior decoration, but these are almost always to illuminate walls and ceilings above us which then bounce light down to the contents of a room. Also, some techniques in photographic and studio lighting do use some up-firing lights, but these act as shadow fill, never as the primary light source—except when an unusual look is intended.)
To the extent that we expect a certain left–right orientation, this will almost certainly be predominantly a matter of consistency with prior graphical user interfaces. The near-universal standard of a top/left lighting metaphor goes at least as far back as the original Apple Lisa/Macintosh which cast its 1-bit, 1 pixel shadows to the bottom/right.
The fact that people of near-identical cultures can natively integrate either left-hand driving or right-hand driving suggests to me that there's no inherent reason why it needed to be one way or the other. Had the first interfaces begun with a top/right lighting metaphor, we'd probably be all as native to that as British people are with right-hand-drive vehicles.
Upward-firing lights are aimed at the roof to spread the light and create more ambient light. So is primarily used as a down-light by reflection anyway.
I'm not sure the left/right shadows are a learned thing from user interfaces. As a kid, long before I got my first computer I was drawing 3D forms. Shadows always was drawn as light came from the upper left.
Maybe this is because I draw with right hand so prefer light to come from the left so I don't shadow my hand?
Now you've got me wondering if the left/right shadow preference might actually be rooted in other natural phenomena besides human dominant handedness, like something with the rotation of the earth a.k.a. sun setting in the west.
(It's true that upwards-firing lights are often used in interior decoration, but these are almost always to illuminate walls and ceilings above us which then bounce light down to the contents of a room. Also, some techniques in photographic and studio lighting do use some up-firing lights, but these act as shadow fill, never as the primary light source—except when an unusual look is intended.)
To the extent that we expect a certain left–right orientation, this will almost certainly be predominantly a matter of consistency with prior graphical user interfaces. The near-universal standard of a top/left lighting metaphor goes at least as far back as the original Apple Lisa/Macintosh which cast its 1-bit, 1 pixel shadows to the bottom/right.
The fact that people of near-identical cultures can natively integrate either left-hand driving or right-hand driving suggests to me that there's no inherent reason why it needed to be one way or the other. Had the first interfaces begun with a top/right lighting metaphor, we'd probably be all as native to that as British people are with right-hand-drive vehicles.