People in TV shows are paid (via the production company) because the TV broadcaster is paid, because it sells advertising during the show. Advertisers pay because the broadcaster can demonstrate that a lot of eyeballs are watching, and they commission programmes that many eyeballs enjoy watching. There can also be direct sponsors, merchandising, franchises, etc.

People in livestreaming are paid because their platform is paid, because it sells advertising during the livestreaming. Advertisers pay because the platform can demonstrate that a lot of eyeballs are watching, and they support livestreamers that many eyeballs enjoy watching. There can also be direct sponsors, merchandising, franchises, etc.

If your question is not the economics but the social aspect (why would someone watch livestreaming?), the answer is that either the livestreamer is doing something cool that people like watching -- and you can be as judgemental about that as you can be for TV shows ("they're watching Real Housewives / a guy speedrunning Super Mario Bros / HYROX championships ??!?!?"), and there is now also the live-service aspect, which you've experienced for decades in the form of talk radio with callers, and DJs taking live requests. Also, in the early days of the internet, there used to be forums that would host watch parties, so everyone watched the TV at the same time and commented in a thread to get the metacommentary and it built up a watching community. That is now integrated into the livestreaming, and the chat sidebar is visible to everyone watching, approximating a live audience, who can let their thoughts be known in a slightly more nuanced way than just cheering/booing/clapping.