Switching cloud providers at Netflix's scale is an enormous multi-year technical undertaking, plus they would be exposed to the risk of two cloud providers instead of one while in transition, plus there are a lot of unknowns in switching providers at that scale which have all been ironed out over the years at AWS, and which could negatively impact customer perception of their product. Plus, as one of the showcase examples of cloud computing success that AWS loves to refer to, I'm sure that Netflix has some sweetheart deals that are difficult to compete with. It would take many many years for "dollars diverted from our competitor" to come close to "dollars spent executing the transition", even with the assumption that a successful transition is possible.