The real reason is that bandwidth is dirt cheap, if you know what are you are doing at scale.
For 'hobbyists' there is a lot of complexity with setting up your own streaming infrastructure compared to just using YouTube or Twitch.
Then for media companies who want to own it, they can just buy their own infra and networking which is outrageously cheap. HE.net advertises 40gbit/sec of transit for $2200/month. I'm oversimplifying this somewhat, you do have issues with cheap transit and probably need backups especially for certain regions. But there isn't much of a middleground between hobbyists and big media cos.
For piracy (live sports streams), I've read about https://en.wikipedia.org/wiki/Ace_Stream being used for this exact purposes FWIW. This was a while back but I know it had a lot of traction at one point.
This is basically the answer.
Minimum latency broadcast forms a B tree. A tree is by definition not peer to peer. The number of branches per node is upload speed divided by bandwidth of the stream. This branching factor is extremely low for residential internet with asymmetric high download and low upload speeds.
Once you add malicious adversaries in the P2P network or poor network connectivity, each client will need to read multiple streams via erasure encoding at once and switch over, when a node loses its connection.
This sounds like fodder for a really interesting blog post tbh. I would read it.