Actually it’s mostly because we’ve spent the last few decades turning math into compression. Starting with MP3s in 1991 there’s a whole history of amazing improvements to audio and video codecs. We’ve made extremely rapid progress even though the process has frequently been weighed down by ridiculous software patents.
But that said, there’s nothing stopping video streaming services from offering higher bitrates if they think it’s worth it. After all, the FCC standard for broadband is 100Mbps, not 16Mbps. That’s a lot of headroom if you want better quality.
It really is mind blowing to see how much smaller a file with newer compression can be. Had to convert some videos to a codec that would work on my kids chromebook recently. What was a 300 meg video ballooned up to 2 gigs. One of the videos was 130 megs that jumped up to 3.2 gigs. Time lapse video, so I'm sure it had some of the sweet spots for compression. Still, was dramatic how much a modern codec can get that down.
Because they compress all their streams to hell to save a few cents and to fit through people's shitty internet connections.