Netflix runs a CDN for the video nodes, but runs on AWS for everything else (eg login microservices). The CDN, which includes hardware deployed inside of ISP), represents a huge capital outlay (over decades) and is a competitive advantage for delivering large amounts of data around the world.
For their use-case it's probably as cost effective as it needs to be. Bring all that cloud hosted infrastructure in house and then you've a whole new set of costs to deal with that aren't present in their current arrangement.
