Because stealing other people's content is a crime as it should be. Have you ever made a living producing, writing or creating content be it books, articles, videos, movies, etc.? I don't think you would ask why if you had.
I don't care what the streamers charge. If I think it is too expensive or simply not worth what they are charging, I pass on it. I don't steal it. There is no excuse despite the insane rationalizations I frequently read when this kind of a topic comes up. The irony is in forums like this there are large numbers of participants who make their living producing one kind of content or another. Yet, when it is content owned, controlled or otherwise distributed by someone else or a big corporation they think stealing it is okay because they don't like the price.