Why does Apple offer full "native resolution" at all if it functionally locks buyers out of it?

You seem to assume that when the Mac is running in Retina modes, that it isn't using the resolution of the display.

I'm currently running a 27" 4K monitor in Retina "looks like 2560x1440" mode. This involves three resolutions:

• The System Settings > Displays, or "UI looks like", resolution is 2560x1440 pixels.
• The Mac draws on a canvas that has 2x as many pixels in each direction – a 5120x2880 pixel (5K) canvas.
• The Mac downscales the 5K image to 4K to get the picture that it sends to the monitor.

This is not the same as "2560x1440 (low resolution)" mode, where the canvas only has 2560x1440 pixels. (That mode is hidden away in the "Show all resolutions" view because most people have little use for it.)

If go into Displays Settings and select Retina "looks like 3008x1692" mode, the Mac draws on a 6K canvas before downscaling things to 4K for the display.

Ed-Finnerty wrote:

I appreciate that, but I don't know what that means in the context of my Air. All I see is 1470x956 set as "default," but when I select the advertised native resolution of 2560x1664, the UI elements are unusably small.

Applications have traditionally sized things in terms of pixels – and assumed that more pixels = more workspace. So physical sizes of text and objects have varied depending on the PPI density of the display.

• 24" 1920x1080 monitor has about 91.8 PPI.
• 27" 2560x1440 monitor has about 108.8 PPI.

So text would be visibly smaller on the second monitor than on the first, and the human sitting behind the screen would be expected to adjust.

A 13" M2 MacBook Air has a screen with a pixel density of about 224 PPI. So something that is 100 pixels wide on that screen will be physically a lot smaller than something that is 100 pixels wide on the others.

If you selected Retina "like 1280x832" mode, text and objects would be sized "as if" you had a 112 PPI screen – that is, to say, they'd be about the same physical size as on a 27" 2560x1440 monitor. Most applications would draw on a (2x1280) x (2x832) – or 2560x1664 – pixel canvas whose resolution exactly matched that of the LCD panel. So that mode would be a good match both for the screen's native resolution and for your eyes.

The default squeezes out a bit more "workspace" at the expense of somewhat small text and object sizes.

I could be mistaken, but I believe that in the early days of high-PPI displays, many Windows programs ignored the Windows scaling factor, and would display unusably small output. On Macs, Retina-aware applications identified them as such. So if the operating system got a drawing request from an application that was not Retina-aware, it could assume that the application thought the Displays Preferences resolution was the real one. The OS was then in a position to transparently adjust the legacy application's drawing requests so that the legacy application drew stuff at the right size, and in the right place, even if it wasn't able to provide more detail.

Basically, when it came to high-PPI displays, Apple paid more attention to backwards compatibility than Microsoft did.

Ed-Finnerty wrote:

That's a plausible reason for a default behavior matching my experience. I just don't understand why they can't allow someone to select the advertised maximum resolution and retain a usable interface.

Commercial book printing equipment has much higher resolution than any display, even a Retina one. So a book publisher could print their books in all 2-point and 3-point type, the better to use the printing press's "maximum resolution". Yet publishers consistently choose to use much larger typefaces. Could this have something to do with the fact that human vision is sensitive to physical size?

There is an inherent tradeoff between the physical size of a display, the physical size of displayed text & objects, and "workspace" (or the amount of "stuff" you can cram onto the screen at one time). This is true whether you are talking about Retina scaling modes or about a global scaling factor. The idea that more pixels should always be used for more workspace simply doesn't work for high-PPI displays.

Why even offer that resolution for the built in display at all in that case?

If it wasn't in the list, we'd probably be getting posts on these forums asking why it wasn't there.

But as to "why" Apple includes it or excludes it, you'd have to ask them.

Apple does what it does for reasons known to them. We can not speak for Apple here as we are volunteer users like you are.