I think there are a number of reasons why Apple specifically hasn't done this. In addition to what others have already mentioned (demand, segmentation, profitability, etc), another factor would probably be difficulty with the overall design.
Part of why Apple's products are often praised for their design is that they do a few things really well and focus on those things, instead of trying to do absolutely everything. Consider the iPod, the iPhone, Apple TV, etc -- they're all pretty focused on doing certain things and Apple's really polished the experience. The Mac desktops and laptops kind of stretch this by allowing more things, but they still largely try to focus the user into certain workflows, via the plethora of apps that come standard with macOS and the vendor lock-in that they push.
Making a phone that can also be a full computer goes against these design principles. Apple's closed the gap a bit in recent years by making macOS and iOS a bit more similar than they used to be, but they're still pretty different. If you're on a M1/2/3/4/etc processor laptop and you've tried using an iOS-specific app (not ones that's designed for both phone and desktop) on it, you'll see some of those differences (interfaces tuned for touch are weird with a mouse, things are sized wrong for desktop, file restrictions can be weird, keyboard input can be lacking, etc etc etc), and it's not enjoyable. Going the other direction, the first thing that pops into my head is: how in the world would the mac desktop be represented on iOS? I'm someone who keeps a lot of files on his desktop, grouped in different sections of the screen for different reasons, and I have no idea how that would be represented on a relatively tiny phone screen (at least in a way that didn't destroy my intentional groups). There are other aspects of macOS that would prove tricky to have analogs on a phone screen, too, but this reply is already getting so long that very few will read it...
Now that's not to say that it's impossible. In fact it probably isn't. But there would be compromises (and those compromises would be on top of the compromises already present in iOS/macOS). To do it well, it'd be a much bigger project than most people realize. It's not just changing a few options and letting us use our phone that way. It'd be more akin to designing the first iPhone. Note that it's not just Apple who hasn't done this yet. Literally _no one_ has done it well yet. I truly hope one day Apple (or someone else, even) does it well, since that'll be a glorious day. But it'd be a huge project, so I'm not holding my breath.