That's what's blowing my mind. You could get a Mac for the same price or less. Everything you can do with a iPad Pro, you can do on a Mac, especially now with the emulator playing iOS apps. The only thing it doesn't have is the touch screen. So what is the reason to buy these?
There are many things an iPad can do that a Mac can't...
Even if you don't consider artists (which is an important market), you cannot use it to take handwritten notes, annotate document with a pencil... And even if you don't have a pencil, you can still use it to read as a book, thanks to aspect ratio and possibility to use it in portrait mode, including while standing (and it's much lighter if you need to hold it). Only when you write you need a keyboard and even then you can easily remove it. And then there is the fun factor (and I guess touch is part of the fun factor).
When you like to use something you are more willing to do things.
I see this with electric scooters for instance, they are much more fun to ride than a motor scooter or even a car, although those are more capable...
That's why people would like to combine the 2, since it's technically possible.
There are essentially 2 reasons:
1. saving money by just getting one device
2. (probably main reason) convenience of having to carry/manage only once device that can do both (+ cellular for iPads)
But that's not in Apple's interest...