I agree we know there is no technical reason they cannot do it, so it is an intentional decision. But I don't think it's as nefarious as you make it sound.
My outside observation is that:
Apple views the desktop/laptop MacOS experience as the "old" way of having a UI dating back to the Xeros PARC. Everything since then (Windows 3, Windows 9x, Windows NT, 2000, XP, 7, 8, 10 through current, System or OS 1-9, OS X era, MacOS through current) is all part of one conventional UI paradigm. Specifically, that UI is a rough skeuomorphism of mid-century white-collar work desks. There is a "desktop," there are "files" organized by "folder" and you can move things around and top of each other like paper, or you can put files/folders away in a shelf to not see it on your desktop.
On the flip side, Apple wants iOS and iPadOS to be something totally new. They want to leave the above old UI paradigm behind and invent a completely novel UI paradigm. To merge the two would concede that they cannot invent something totally novel.
But we've been working under the old UI paradigm for 40 years now. That's multiple generations of workers used to it. So Apple is swimming against the flow in a major way. They've been forced to concede and sneak-in some "old" ways of doing things into iOS/iPadOS (e.g., "Files"). But they're still pushing ahead to try and make it so their novel UI completely obviates the old way of doing things.
To give credit where credit is due, the iOS/iPadOS UI is really good for a ton of uses. But it's has not completely obviated the old way of doing things, and at the moment it cannot.