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.
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.
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.