It seems like they’re leaving a lot of money on the table. I don’t think a significant number of people would jump ship if Twitch updated their policy to give 99% of donations directly to streamers while taking a 1% cut for themselves.
There might be some initial grumbling, but it would probably fizzle out in a week when people move on to other things.
Twitch takes a nearly 30% cut on bits and about 50% on subs. They don’t take a cut on charitable donations because that’s not legal.