Earlier quoted context omitted.
They're making $200 million / month based on their S-1 filing. I'm sure they've done the math and the effort to cost savings here simply isn't worth it to them.
Or, more likely, in the beginning AWS is what their developers knew and were familiar with, and then as time went on it was easy and convenient to just spin up "one more instance". Fast forward to today, and now it would be a serious undertaking with serious risks to move off AWS, not to mention the costs of building up the staff and assets to reimplement their requirements in parallel of AWS until reasonably confide…
The “whims” of Amazon’s pricing are no more unpredictable than the pricing “mood” of your colo or your hardware vendor.