Apt is, at it's core, a dependency engine. These apps have no dependencies- It's the whole app package.
Also- Why wouldn't they base it on the already successful package distribution platform they have for iOS devices?
In short, yes.
Yes. (They already have rolled their own, why would they switch to apt?)
No need to use apt. Apt is great for handling dependencies, but when it comes to mac apps there are just two parts: OS X and the app. No dependencies between apps and no dependencies on libraries not part of the OS. Nice and simple.