It really isn't cheaper. Netflix pays $1bil/yr to aws. The only advantage streaming providers have is that you're already paying for an internet connection. But TV over internet solutions have been a thing for a long time so even this doesn't really matter. At the end of the day it's just bits going through a plastic tube from one computer to another. If anything, the 1 to 1 on-demand aspect of streaming would make this the more expensive option.

But the networks didn’t (mostly) own the traditional TV stations. Streaming cuts the middlemen out of the loop which is probably a bit cheaper though overall.