1. Bitcoin is a drop in the bucket. There are many, many, many gamers out there. Just look at the CoD sales numbers.
2. Developing a GPU is a very long process. Definitely greater than 1 year.
3. Current ATI GPUs are already very good at this. Making them better would entail sacrificing elsewhere.
4. Bitcoin is unstable and new and has a questionable future. If the exchange rate falls, mining hardware interest will fall as well.
5. No company in their right mind would invest in hardware tailored to a small niche with an unstable future.