That was probably thought about at some point but the thing is that Apple makes way too much money as it is to try anything, because they don't have to, at least for now. I'm pretty sure Apple's leadership knows about this possibility and there are even some prototypes most likely but they conveniently rejected the idea because it would mean selling less devices. Even if they price the combo higher than their laptops current it would still make less money overall (especially since they already got the prices so high there is very little room to push for more, as falling mac sales attest). This is basically the same reason the iPad never got a multi-user mode even though it should be relatively easy to implement since they have all the basics in macOS already.

I think Apple is blinded with money and is being way too complacent, ignoring that they are in fact losing customers to more modern 2/3-in-1 devices, especially in the young/students demographics, just like they lost customers because of their lack of interest in PC gaming. And since they make it extremely hard to integrate their stuff with other products, people won't renew their Apple hardware overtime, since it's an easy decision considering for how much they sell their stuff.

And I think they are way too proud to admit that Microsoft was actually right with their Surface, for most people it is actually a much better device since a tablet isn't that useful in the end. If all you want is entertainment, a cheap Android tablet with a decent screen is all you need. The iPad Pros as they exist, are way too overpriced for their real capabilities and at this point are terrible value.

If Intel or AMD successfully make a competent power efficient chip in the near future, they will be only Apps left as an "advantage". Considering many of the powerful iPad apps are stripped down versions of the fully featured versions available on "legacy" computing platforms, that's not saying much.