Isn't that like saying that Apple stopped selling the "toaster" Macs so that Apple could sell both a computer and a display?
No.
Longer answer, you could buy lots of third party displays that worked perfectly well with Macintosh computers (and you still can) so that also means Apple can fail to sell you a display by not integrating it.
The iPad is something like the "personal digital assistants" from ancient times.
The iPad is absolutely capable of running Mac OS, but it's artificially restricted from doing so, in an effort to make you buy Mac OS. And there are Macintoshes which could easily run iOS, but they don't let you do that.
This distinction was created artificially and intentionally both to enforce a certain style of use and to sell more devices. The first thing is a marketing decision, that's understandable and even reasonable. The second thing is also a marketing decision which is also understandable, but repugnant.
There's no reason why Apple could not have simply let you run in both modes on both kinds of hardware, allowing you to choose, and to provide user interface standards for both types of interface — and allow apps to implement one thing or both. And there's no reason why they can't switch to doing that.
The question of whether they should be forced to do so is a lot more complicated, and even I'm not sure they should. But it's telling that Android is embracing Linux as the devices continue to get closer together, while Apple is still trying to distance their platforms from one another. But they're ultimately doing their customers a deliberate disservice. As Linux continues to improve, perhaps more slowly than it "should" but still doing so, there becomes less reason to stick with their artificially limited forced duality.