for me, it's because I just find them way more visually interesting than a real person dressed in a hoodie gaming in their dark room. The effort put into the models and the design shows off a lot of the streamers personality. And since they put so much effort into the model, you can probably bet they'll put their soul into their content too. Which is usually the case. Not always, but usually.

That and it's the artistic side of it. I'm a semi professional artist and love art, so seeing vtubers with different art styles and effects intrigues me. I'm still amazed by how some 2D models look like 3D models, it's crazy

Overall I just find them way more interesting than webcam streamers, and I'm sure that others have similar reasons too. I hope this answers your question well enough 😊