Because watchOS and tvOS are both based on iOS, apps for those platforms can share code, be packaged as universal apps, and be sold in bundles. Not so macOS, which uses AppKit instead of UIKit, the Mac App Store instead of the iOS App Store (and it's offshoots for TV and Watch), and generally has to be planned for and executed separately. But just because that's the way it is doesn't mean that's the way it's always going to be.
Mark Gurman, writing for Bloomberg:
Starting as early as next year, software developers will be able to design a single application that works with a touchscreen or mouse and trackpad depending on whether it's running on the iPhone and iPad operating system or on Mac hardware, according to people familiar with the matter.
Apple currently plans to begin rolling out the change as part of next fall's major iOS and macOS updates, said the people, who requested anonymity to discuss an internal matter. The secret project, codenamed "Marzipan," is one of the tentpole additions for next year's Apple software road map. Theoretically, the plan could be announced as early as the summer at the company's annual developers conference if the late 2018 release plan remains on track. Apple's plans are still fluid, the people said, so the implementation could change or the project could still be canceled.
It remains to be seen if, when, and how Apple would roll out universal iOS/macOS binaries. (How, not what, is always the truly interesting part — UIKit for Mac, anyone?)
What's clear is that the company has been doing things along these lines, internally, for years. The iWork apps for Mac were burned down and then built back up using the engine from iWork for iOS. Photos for Mac was bridged from Photos for iOS. More recently, Apple has been merging the teams and work on the core technologies behind their apps while maintaining the separate user, context-appropriate, user experiences.
In other words, this is nothing new. It's the next progression down a long road that, like with tvOS and watchOS, will let Apple and hopefully developers work wider and more efficiently.
For Microsoft, shifting to universal apps was a way to shed legacy baggage and encourage support for post-PC devices. For Google, bringing Android apps to Chrome let them tap into native functionality and performance.
For Apple, it lets the massive iOS platform help pull the Mac platform forward. So, for example, we don't have to live years without bubble effects on macOS. (Tragic, right?)
Universal iOS/Mac apps wouldn't be about surviving. It would be about thriving. At least if Apple is responsible enough to enable new and better pricing options for developers — including per-platform tiers and bundles.
WWDC 2018 kicks off this June. Happy holidays.