[GreedClarifies]

What does Amazon use for their ads network?

My guess is that it just isn't competitive and so Twitch can't monetize.

Or maybe it is the demographic. But the ads network is my first guess.

[hoofhearted]

They use the Amazon Advertising Network.

Their advertising machine is third largest revenue generator next to sellers fees and AWS.

[GreedClarifies]

IIUC their advertising machine isn't quite as general as the networks of Google and Meta, and gains a large portion of its revenue from ads on Amazon Retail, which may not be competitive.

It's a guess as to the root cause.