Same reason why kids have paid for Transformers merch, Star Wars merch, band merch, etc.
It's a brand, they like it, they want to be reminded of it and show their love of it off. It creates an "in group" which is socially valuable. Streamers are nothing special in that regard.
I had football jerseys with my favorite player's name on them growing up and I'd look up to my birthday to see if I got one or I had to wait another year. This seems like an arbitrary decision. I don't see any difference in buying a jersey of my favorite player or a kid now getting a t-shirt merch of their favorite youtuber.
I think the implication is that if a kid buys a toy they will have something tangible that they can play and interact with, but tipping/donating to a streamer doesn't provide that.
It's a brand, they like it, they want to be reminded of it and show their love of it off. It creates an "in group" which is socially valuable. Streamers are nothing special in that regard.