(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.
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?
Off topic but I like how you've referenced that infamous IRC meme in your HN handle.
#20 Fool Yourself into Seeing 3D -https://books.google.co.uk/books?id=K6bjvFUcedgC&pg=PA57&dq=...
(It briefly mentions Susan Kare's work who designed the first 3D buttons in Windows 3.0 -- 1990!)
There's another effect going on here which is visual affordances: not only does the button "pop out" but we immediately see it as something we can push. That's from the psychologist James Gibson in 1977.
There are a ton of cognitive quirks that are useful to know when designing interfaces. The pop-out effect is one of my favourites.
My CSS-fu is very basic, so it might need further tweaks to make button presses look right.