For the viewer: Playing take a lot of energy, watching is more passive. Especially like horror games that raise your blood pressure / heartrate. Also, not everyone can afford games, some are console only, and even PC games have hardware requirements that people don’t have, because people either have a potato computer, or just have smartphone only. Also, games are hard, watching a streamer dying is kinda funny.

For the streamer: Socialization (even tho its kinda one-sided, they can still read comments / live chat), and most importantly, money.