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This is only part of the reason. The other reason is that Apple doesn't actually do fractional UI scaling, they only ever render into a framebuffer at whole integer multiples of an internal "native" resolution and then scale down this framebuffer to the actual monitor resolution afaik (they still render natively to this multiplied resolution - no doubled pixels or anything like that).

This results in less crisp output than you would get if you actually rendered vector based UI at the exact monitor resolution or if you natively rendered bitmap art 1:1 with the monitor resolution.

This approach makes sense when you're only using Apples monitors because they can optimize their internal "native" resolutions for the handful of actual native resolutions they offer, but it falls apart when you start plugging in external third party lower dpi monitors.

(I believe this is also the same approach that Gnome 3 uses to do (experimental, with a toggle) non integer multiple UI scaling.)