Ask yourself this question: why do people listen to recorded music, when they could derive entertainment by just playing the music themselves? It's because people who are highly skilled at playing music are more entertaining to listen to than the act of playing music is for even many amateur musicians.

The same principle applies to live streams of people playing video games. Most can't just replicate what they do regularly without hours of practice, especially speedrunners who spend hundreds if not thousands of hours attempting to optimize their runs and practicing glitches and sequence breaks not easily done by an amateur or casual gamer.

The vast majority of these runners do this as a hobby, developing these skills for free and providing countless hours of free (at the point of access) entertainment for hundreds of thousands, if not millions, of young people with a lot of time to kill and who can't afford cable or a streaming service. They have consistently and regularly raised money, eventually hundreds of thousands of dollars, for charitable causes, for over ten years, entertaining potential donors through a telethon-like format. Some have since branched out to accept sponsorships from esports-like organizations to continue running games for a living.

Regardless of the motivation or level of investment, these people produce something of value, and very cheaply. That is why Amazon bought Twitch and pours money into this bullshit.

All that said, I have no fucking idea who any of the actual "big name" streamers are who don't do speedruns.