Why not put the A14 or A15 from the iPhone in a MAC as well? The iPhone chip is powerful enough for most people and Apple could introduce an even thinner and smaller ultrabook laptop, the 11” MacBook?
Remember that the iPad Pro now uses an M1 - so Apple could easily produce an 11" MacBook, if they wanted, without "downgrading" it to an A15.
...and while I'm not arguing with the people who might want an 11" Air, I suspect that
Apple's line would be that you should buy an iPad for that.
More interesting would be the ability to run MacOS on the (M1) iPad Pro.
Besides what's been mentioned, A14/A15 have fewer GPU cores and don't support an external display.
iPad (even pre-M1) supports an external display (hence the existence of
this adaptor) for mirroring
or media playback, independent of the main screen - so the hardware is quite capable of driving two displays, although iOS/iPad OS limits what you can do with them.
Macs also have a lot more going on at any given time. iPhones lack windowing and are not designed to have apps running in the background (like doing a render while you are browsing websites). The 2x4 layout of phone chips is just not adequate for a computer workload.
iOS/iPadOS is a fully multi-tasking operating system, and the iPad/iPhone rely heavily on background tasks to do what they do. iOS places artificial restrictions on how
3rd-party apps and the user can create background processes - but that is a software restriction - and one that has been slowly relaxed since the first iPhone, as mobile processors got vastly more powerful).
Let's be clear - there's no
point in Apple making an A-series Mac when they could simply make an M1 Mac the size of the 11" Air (or a Mini the size of AppleTV) with much less compromise. However, it would be perfectly
possible given that Apple
have already done it in the shape of the Developer Transition Kit - an A12Z processor in a Mac Mini housing running MacOS. During the brief period for which Apple actually maintained support for it, and allowing for the fact that it was never destined to get out of "alpha", that did a perfectly credible job of running MacOS. Yes, there were restrictions - but you could say the same of the 12" MacBook in its day - (e.g. no Thunderbolt, and you probably wouldn't
want to run a 4k display, multiple virtual machines or do heavy multitasking on that CPU/GPU, even if you could).
Some of the arguments here are reminiscent of the "why Apple will never switch to ARM" that seemed to suggest that ARM-based processors lacked some sort of secret sauce that "real computers" needed (...in complete denial of the benchmark results for the iPad Pro). Any processor that can cope with the iPad
user interface is going to be good enough for an entry-level ultra-mobile.
