it is like asking why doesn't the barber shop fix my computer!!!
Amazon is not in the business of creating exchanges, and their business is not even remotely related to exchanges. so why would they want to get into this business out of nowhere?
they may however end up creating their own coin (Amazon-coin) and add one more useless coin to the sea of altcoins.
Yes, that's what I thought. They could create a token on eth or something and accept it as means of payment. If the fees will be cheaper than when one is transferring local fiat and it is automatically converted into usd (and accomplishing this aim is not so hard), it might even work. However, I think amazon showed its attitude toward cryptocurrencies some time ago by sort of accepting (but not really accepting) bitcoin as means of payment when the coin was already very popular, but had no speed or fee problems yet. I suppose it means that they don't really care about it and keep accepting fiat to be safe from potential major fluctuations.