Plenty of ebikes come with hub gears.
My Riese and Muller has a Rohloff.
Cheaper makes tend to use the eight speed Shimano Nexus, which is also pretty bomb proof.
I'm guessing drum brakes might lack the extra stopping power needed for a heavy ebike, although marketing is probably the main reason.
Drum brakes are seen as old-fashioned.
As with most things ebike, push bike criteria applies.
Drum brakes are rarely fitted to push bikes.
Hub gears are dearer than derailers, so makers trying to hit a price point don't fit them - same as with push bikes.