As much as that's true, Apple has been trying to dig deeper into "services" over the last decade. App sales, music sales, movie sales, cloud services, anything they can do to expand their software ecosystem to make it more versatile and useful on hardware they never produced.
If in some conceivable future Apple could make enough money on the "pro" market looking for OEM-style rigs to fund development of macOS through the macOS App Store, or through OS sales, they would probably at least consider it.
The problem Apple has is success. If you're managing a team of top talent and you have two products, one of which rakes in hundreds of billions, the other which is mere tens, which team do you think gets the best people?
It must take a lot of determination to keep really great people on the macOS team instead of poaching them for iOS. Apple's strategy of using the same OS core for both platforms probably helps in that regard: They can put their best people in the middle without concern about their full potential being wasted.
It's a really tricky spot to be in. If Apple had no phone business at all they'd be a tiny company, but they wouldn't have the same stress about where to place people.