And why (after 4 years and counting) have they not done so anywhere at all?
Because Bitcoin
can't compete with VISA, MasterCard, etc. due to scalability issues. THEY just do not worry.
Can't may not be the right word choice here. BitPay already has more than 8,000 merchants paying a fee far lower than traditional payment processors. Bitcoin will only become more and more simple to use with time and acceptance. Remember we are still in Beta and "real" infrastructure is still very limited.