The question you should be asking is why is streaming video so expensive that DVD (shipping little plastic discs around) is cheaper than sending bits over a wire?
Because it's the second stupidest deliberate misuse of computational capacity to artificially create digital scarcity since Bitcoins.
The correct way to distribute large files like movies online is to copy the bits as locally to the endpoints as possible, and cache them pervasively at all levels of the network. Nothing would need to be sent more than once down any given cable. It would be fast, cheap, make use of the Internet as it was designed to function, and give us near-unlimited bandwidth.
But that would mean that those bits don't become artificially scarce and can't be tracked and audited by the media companies for copy-protection purposes. So instead of copying, we stream them over and over and over again, generating terabytes of needless, duplicated data traffic, and creating huge bandwidth storms that suck all the capacity out of the Internet.
tldr: Video streaming is expensive because it was designed to be. It wasn't designed by or for you, and it doesn't benefit you.