Scaling isn't even relevant. It makes sense to talk about scaling pixel artifacts, but not vectors one. The font is 8pt or 20pt, you can configure that on your own. 1:1 scaling is just some arbitrary and artificial fixed pixel/letter seen.
A 24" 4K screen (basically a 24" imac) had text the same size as a pre-retina 24" imac, they just used more pixels to render that text.
In any case, Windows is just using a scaling feature. But when you scale, you are losing workspace in exchange for sharpness.
When 4 physical pixels translates to 1 logical pixel, you've actually lost 3 pixels of potential real estate on a screen size where your eyes can actually discern those pixels.
On a 15" 4K monitor, losing that real estate is not a big deal, because >90% of people can't easily read or see anything scaled at 1:1 on a screen that size. When you scale that screen at 200%, you're basically using a 1080p screen with sharper text. You don't gain any real estate at all from the higher resolution.
On a 40" 4K screen, it's a whole other story. The text may not be sharp, but you can have way more windows open on that screen, which makes it easier to multitask. It's like having a grid of 2x2 20" 1080p screens.
-addendum - my visual limit is 1080p @ 1:1 on a ~14" screen, which is why I am fine with no scaling on my 4K 27" screens.
Well that is a user setting, and I tend to think the first retina iPhone is what started the trend.
Also, at certain screen sizes, 1:1 scaling is just impractical due to limitations of the human eye.
My 27" 4K screen is pretty much at my personal limit in terms of what I can handle at 1:1 scaling for my normal viewing distance. Had I gotten a 24" 4K screen, I'd have to set the text scaling to ~125% which would also translate into some lost screen real estate.