Exactly. The AppStore already exists and does updates (either automatically or manually, configurable by the user). The developer didn't have to lift a finger to get this functionality. Imagine sitting down and spending time adding functionality to your application that is already provided for free by the operating system, and then after all that, doing it incorrectly!
Starting from the paid developer accounts, the Apple app store isn't "provided for free by the operating system" and it is a source of endless busywork, fear and suffering, but the argument stands: a professional Macintosh software vendor uses the app store because Macintosh users expect it, so it can be assumed that "properly" publishing new software version to the app store is a sunken cost that should be made as useful as possible.
By "provided for free" I mean the App Store comes with the OS, costs nothing (monetarily) to the developer over the existing annual Apple Developer Program fee, which pretty much all macOS developers pay anyway, and can be counted on to exist on all macOS installations.
