And only quite recently has "billet" stainless steel and aluminium become a "common" material for consumer goods (as opposed to types like die cast which don't have a finish that's usually left unpainted).

Both these materials require fairly advanced techniques to produce at scale. Firstly the actual materials themselves are fairly hard to get good alloys of.

Secondly, they are hard to machine well, especially stainless steel, which really needs extremely rigid machinery and high speed carbine tooling. Not only that, but to machine the shapes of things that would previously have been plastic, you really need CNC, which again, only recently has become economical for many consumer goods.

Scientific instruments and tools meanwhile would be using brass, as it's much more amenable to machining. Milling a brass billet on a manual mill is a joy and results are good, stainless is a misery, and the results are horrible.

And as to why plastic things were so garish a few decades ago? Maybe firstly they were new and it was unusual to even be able to have such colours. Early plastics were ahead of the dye chemistry, so they were much more "natural".

Secondly, it's actually hard to have good silver finishes (you can have grey): many of the high end products were silver, but it was usually a coating which would come off and reveal the milky plastic. White is also hard because UV stabilization was also not as mature, so it would yellow quickly[1]. It was easier to have a solid colour, and the brighter the better in terms of hiding moulding defects.

[1] It also seems only fairly recently that you can get the really deep gloss jet black plastic, rather than the dull, very dark grey like a cassette tape body.

To non-technical people, polished stainless, aluminum, and chrome are indistinguishable, and chrome was utterly dominant in the automotive field before the 1980s.

Automotive yes, and standard profiles like tubes, but not smaller consumer goods because it's very expensive to tool up for a chromed item due to the finish needed on the substrate (without CNC, remember). For example a chrome case for a telephone handset would be brutally expensive compared to plastic (until metallised plastic was developed).

I think it's high time for "garishness" to make a comeback, at least in the realm of handheld electronics. It's exasperating, the belief product designers have that every phone or remote control has to be of such a color that one can't tell the devices apart or spot one of them from across the room.