Part of it is that carpet and plywood are fabricated materials. Prefabricated plywood and wall-to-wall carpeting weren't really things until ~1930s. The older materials seems more luxurious now, but that's because the newer materials have become cheap. It's like how aluminum was once more expensive than gold, but is now ubiquitous because the material itself has become much cheaper.
https://www.vam.ac.uk/articles/a-history-of-plywood-in-ten-o...
https://www.thisoldhouse.com/flooring/21017998/all-about-wal...
Tiles were ~1" thick: https://archive.org/details/TheBookOfVermontMarbleAReference...
Using an 1889 price book, it cost ~$0.25/ft2 of tile. Add $0.70/ft3 for shipping to St Louis, which is $0.06/ft2 of tile @ 1" thick. That's ~$0.30/ft2 total in materials. https://archive.org/details/pricelistrutland00verm/page/16/m...
50' x 50' x 10' building, (floor:(50 x 50) + walls:(4 x 50 x 10)) = 4.5k ft2, which @ $0.30/ft2 is $1,350 of marble.
For comparison,
- Average annual wages look like ~$700: https://babel.hathitrust.org/cgi/pt?id=mdp.39015002738212&vi... - A 50'x50' 2 story house was probably $4-5k: https://archive.org/details/gfbarber1905/page/n20/mode/1up
So adding the marble interiors may have been 25-33% of the price of a house. If the average US house today is $400k, then that's $100-130k for the marble interior.
Heavy metal doors with perfectly smooth-operating hinges. Marble everywhere—floors, counters, stairs, [edit: hell, even the walls!], everywhere, and this was not in an area that mined marble, it was surely imported from at least several hundred miles away, and likely much farther. Thick, ornate brass doors on the PO Boxes. Serious- and heavy-looking metal light fixtures. I loved going there as a kid. It seems silly, maybe, but that post office felt magical.
Similar story for older libraries (including the one in that same town), older university buildings (ditto), older bank buildings even. They're all so nice to be in, and embody a confidence in some kind of permanence and continuity.
Now all that shit's in strip malls or buildings that are otherwise intended to have a 50-year lifespan at most. Cheap low-pile carpet on plywood, comically fake ornamentation if they bother to have any at all.
What's so weird is that we built basic public buildings like we were rich, back when we were, relatively speaking, paupers, but now that we're wildly rich we build everything like we're paupers. It'd be inconceivable for anyone to get half the budget it'd take to build a post office like that one, for a new post office building these days.
Hell, even my Grandparents' cheap, small post-WWII house in a cheap rural town had details that are rarely found outside luxury homes today—the heavy solid-wood front door and extremely solid-and-smooth-feeling metal doorknob, nice metal switch-plates and heavy-feeling, satisfying switches, that kind of thing. I bet you'd have to special-order a storm door to get one with more than half the metal in it that theirs had and it'd cost a fortune, and god, all that stuff felt so nice, and held up to years and years of use without being the worse for it. Their whole working-class neighborhood was built like that.