I'm not sure there's much more explaining needed than multiple posts and screenshots.
macOS can do 72 dpi (1x), 144 dpi (2x) or 216 dpi (3x, which I'm not sure has actually been attempted on macOS, so maybe it can't do that).
There were versions of Tiger and Leopard (maybe also Snow Leopard?) that also supported fractional settings, such as 1.5x. You could even go down to, say, 0.8x.
View attachment 1846800
This support was limited to developers and considered not ready for users, but developers were vaguely told to start preparing their apps.
However, this was hard to get right in practice — including for Apple itself. For example, here's TextEdit at 1x:
View attachment 1846803
It's especially hard when you have fractional values.
Fractional support never shipped, and instead, around the same time, the iPhone 4 appeared, which supported 2x scaling. Then some later iPhone (the iPhone 6 Plus, maybe?) did 3x scaling. This is much easier, as you can, when in doubt, just render a logical pixel as 2x2 or 3x3 physical pixels.
Windows has since the 1990s supported a similar feature, although it never worked really well. It also didn't support different dpis per screen for the longest time (until 8.1, I believe). In recent years, though, it has gotten a lot better.
Going back to the original topic: suppose you
do put a 4K display in a 16-inch laptop. On Windows, this means it'll probably default to 250% scaling. For apps that do scaling well (which is an ever-increasing amount), it'll look great. (Although, personally, I'd argue it won't really look noticeably better than what Apple does — just put a display in there that's natively around 220 ppi.) On macOS, it won't. What macOS could do is render towards a virtual resolution that's a lot
higher than 4K, then downsample that back to 4K. This will be blurry. Not
very blurry, but blurry nonetheless.
For laptops, this really isn't a problem: Apple will simply put a good native resolution in there. But as soon as you want an external display, this is a huge problem, because
it vastly limits good display choices. If macOS supported something like 1.5x, you could simply get a 20-inch or 24-inch 4K display and use that.
But I'm guesing you'll reply to this with "that doesn't really explain much" again, so maybe I shouldn't have bothered.