Why is the anode on the battery negative?


It has to do with passive sign convention.

The diode absorbs power, the battery delivers it.
Anyway, the point is don’t use anode/cathode. Use mathematics +/- Works in all countries.
Anode/Cathode is a throwback to the ‘good olde days’ when men were men and they used thermionic valves and loads of voltage to give you a nice reminder that the power is on.  Also the smoke rising from the valve and its small red glow also works.
The EHT for CRTs was a pleasure to touch…