I remember when the creative marketing team were questioned about this a couple years ago, they implied that the R8 et al were merely "10 minutes of fame" kind of cars and everyone will forget about them soon, therefore not worthwhile for BMW to make a rival.
Still, I am very impressed at how well the Audi R8 V10 hangs very close to the SLS AMG in a straight line despite being down by nearly 50 horsepower and being heavier.