I don't think they are. Given the same size total audience it's much cheaper for Twitch to distribute one stream around the world a million times than it is to distribute thousands of streams around the world a few times each. Transcoding only has to happen once, each segment only has to be distributed to each area of the world once, etc. This gets more nuanced when you include things like chat, which gets harder as you scale up, but I highly doubt that cost is significant compared to the video feed.
Those large streamers may be monetizing their viewers through other means, but as long as they're not actively pulling them away from Twitch those viewers are still either watching ads or subscribing. And based on anecdotes shared by some of those big streamers Twitch ads have gotten more and more profitable.