When a hype train happens, the streamer is busy doing things. Having to say “oh, can you please go to this web page and click Authenticate” is a barrier that is completely unnecessary.
Also, does it even matter? The streamer doesn’t need to authenticate me to see what’s in chat. But to get the exact same information in a separate web page requires the streamer’s permission. Considering that I watch a lot of streamers, and not all of them should be required to grant me permissions, it’s a pretty serious limitation.
Without broadcaster authentication, I can connect to chat and simply monitor all cheers and sub notifications. I could easily leave a bot permanently connected, and I’d get far more of a view of the streamer’s income than a current hype train will give. This is not about protecting streamer income information.