BMW owns Rolls Royce, so I think they use the 760i engine there.
As for why, I suppose they were developing a high power engine for Rolls and just decide to shove it in the 760 for the sake of it. I suppose it's a lot easier to put it in a similar size car than a small car like M3.
Also the reason why they didn't push more power out of a V12 is because it is not a performance engine. They made it for luxury cruiser for smooth power delivery.
*Oh and there was a high performance supercar with a BMW 6.0 L V12 engine. It's call the McLaren F1.
2011 535i Alpine White, Oyster on black interior, Premium 1 & 2 Package, Magnaflow Full Cat-Back Exhaust, JB4 Stage 2