Well, there is. Say I'm an ISP with X million subscribers. Given a peering with Y GBit of throughput to someone like say, Twitch or Facebook; with multicast,

Well, there is. Say I'm an ISP with X million subscribers. Given a peering with Y GBit of throughput to someone like say, Twitch or Facebook; with multicast, I now only end up with a single load of data for each livestream coming over the pipe. However, this does require direct peerings. Multicast over the Internet is just not a thing. This could technically be built out with IPv6, but nobody's realistically going to arrange for globally-unique multicast space allocations and convince every AS in the world to start peering multicast over BGP for general public use.