I think Apple may have greater flexibility in their system but choose not to expose it as it might confuse their users. Or, they have it there, but it is buggy, so they only expose what they need for their product.
I encountered another instance like this several years ago when I put Leopard on a Fujitsu slate tablet. It was possible to get the digitiser to work so I could use the system without a keyboard.
But it wasn't possible to rotate the display to switch into Portrait mode (wide screen is called 'Landscape mode' - the usual orientation for 4:3 or 16:9 or 16:10 clamshell laptops). In researching this, I encountered the explanation that Apple was concerned users might activate it unwittingly on their MacBooks, and have problems reversing the change due to not being able to navigate the screen while in that mode, while the physical machine remained oriented in the physical world. So they disabled that capability.
It's possible changes to pixel density settings as a continuous range are not built into Apple's display manager, although the range of physical Pixel densities in their panels shows it can be changed. However, they may not have realised it as a smoothly set-able parameter as the others do. Again, they may not have seen a need due to their approach to providing customer experience.
More and more, Macs look to me like "appliances", as opposed to "toolkits". This is a philosophical decision of what to provide, and Apple's success argues favorably for their understanding of their markets. Indeed, much of the rationale for me taking up Macs is because of my hope of tapping into that market.
As a long-term Free Software geek, it's jarring to me, though. At the same time, I can agree with it - from my background in Quality Assurance, I celebrate a manager who said "why should I let my coders write in a language that allows them to introduce memory leaks?"
(Aside: 'Testing' is 'Quality Control' - it happens
after you make something because you're not in control enough to be sure, so you better check. 'Quality Assurance' is
before - it's about making sure you can't go down avenues there is no reason to go down. Most people in the business don't know of or pay attention to the distinction. 'Quality Control' is an admission of the possibility of failure, 'Quality Assurance' is the pre-knowledge of how to avoid failure so as not to waste time and effort.)
So, Apple's approach is... interesting.
I took this Hackintosh with me to a local computer store when I bought an external USB 3 drive to use with Time Machine earlier this week, so I could also plug in an external monitor to verify it worked. I tried with a monitor that could rotate to Portrait mode as that's attractive for coding.
I found I could rotate the display on the external monitor - but the internal panel 'rotated' as well, in the sense that the geometry resized to match the portrait dimensions of the external monitor (9:16) but remained oriented with 'up' still 'up'. Because of the resizing, the internal panel is all-but-unusable in this configuration...