Because they are smoother, more refined, quieter, offer a different style of power delivery which money prefer and life is too short for penny pinching when you are buying a premium car.
Sadly it was sold today but I ran my 530i alongside my parents 530d for years, regularly driving both. The 530d was great and absolutely had its strengths, but fuel economy and... err.. towing trailers aside, the petrol one was just better.
Dont get me wrong, the decent diesel engines are absolutely excellent but to say you can't understand why anyone would chose petrol when, removing the fuel economy aspect, its the better engine choice, is bizarre.
Look at any country where petrol cost isnt an issue. Are they all driving diesel for the 'superior torques'?
Of course not. They all drive the better engine choice when economy isn't part of the equation.