Cost. Switching would cost a ton of time and money, which is probably better spent in securing streaming rights for popular shows, creating their own shows, and marketing to increase their subscriber base.
Netflix is a high profile enough customer that I think they would get amazing discounts from MS or Google if they moved, just so those providers can parade around talking about this massive high-profile defection.
Agree, switching is so costly, and IMHO given Netflix's current profit margin, which is not that high, I doubt they can afford to replace aws with a full in-house solution.