Obviously I don't know the implementation details of how Netflix is detecting rooted devices, but my guess is they're using Play Services's new "GMS Certification".

This involves using signed hardware that helps determine if the execution environment is trusted. So, users can still root the device, but Android + Play Services can still determine if features (such as DRM) can be relied on.

It makes sense for Google to develop these features, especially as it wants to move Android to new devices (Cars?), reduce risk for mobile payments, in addition to giving stronger security features to content distributors. This also lets Android devices compete with security features like Apple's secure enclave.

Honestly, if you are going to root your device you must accept you are not using it in the supported way the vendor intends. Therefore, you should not expect all the features of the supported configuration. I think that's a fair trade-off. Rooting your device is awesome!