You should build one! And then make it so good that Apple will finally copy it for a future version of MacOS :D
I'm only complaining because the ones in the App Store are awful. I just want the native iOS one.
But yeah I dont need a 50th side project.
The calendar ? It's not really a 'clock app' but lets you setup timers and popup notifications. You're right it should be baked in with the system clock.