>>716295618
rendering pipelines are optimized to draw things front-to-back as much as possible in order to avoid redrawing pixels. when starting a new frame from scratch it will try to draw the things closest to the camera first. when drawing it will check each pixel and ask if the depth of the thing it's trying to draw is greater than what is already drawn there (higher depth = further away). if the depth is greater, it will skip it and move on to the next pixel
so if you're staring at an opaque wall 5 feet in front of your face the game won't bother rendering anything behind that wall.
you could also think of it like the inverse of the way painters paint. normally they do the sky, then the background, then the foreground, but that's a huge waste of the computer's time. why draw the skybox first (which will fill the entire screen) when you could draw everything else first, then draw the sky on the pixels that weren't touched

transparent objects throw a wrench in that process because you can't skip drawing things behind transparent objects, including other transparent objects, so large swathes of the screen can end up having to be redrawn again and again and again, blowing out your GPU's fill rate if it happens enough times. this also means overdraw is a scaling issue that gets worse the higher your resolution is. even old ass games can smoke your gpu this way.

this is why devs abuse dithering nowadays, dithered materials are technically opaque so they don't create overdraw, but it looks like shit so they have to use TAA to blend the dither pattern together make it look partially transparent
that said, specialized materials have existed for a long time to solve the hair problem (see: games like the witcher 3 and last of us), but the only explanation i can think of is that this knowledge just gets lost over time because the competent devs get fired