Comment by crazygringo

Thank you. The idea that this is due to ads makes complete sense, as there's a lot of indication that Netflix charges more for ads showing on a TV (more people likely to view) than on a mobile device (usually only one person).

The fact that casting is still supported for older Chromecasts only on ad-free plans makes this the likeliest explanation to me. Netflix doesn't want to be paid lower rates on ads that are actually getting shown on the TV.