There have been other V8 engined motorcycles in the past - they are just too expensive (manufacturing wise) to be able to cost effectively mass-produce for the main stream market.
The most recent "production" bikes of the V8 type are from Morbidelli, a small Italian bike builder, though this was a liquid cooled DOHC V-8 of under 1-litre displacement, not an air-cooled one.
As "cool" as that garage-built airhead V-8 looks, I gotta think that the inside cylinders willl get pretty darn hot if the bike was run in normal day-in/day-out traffic.