If video streaming is so bad, Netflix should not be on the App Store for "iDevice security". If video game streaming is so bad, Twitch should not be on the App Store for "iDevice security".
It's not just streaming, it's interactive streaming where it could well happen that PayToWin / LootBox / etc games can appear.
Now, of course, they already exist on the App Store (and I honestly wish Apple would ban them outright) but there, at least, Apple has some kind of leverage - with xCloud, they have none.
A steaming app (game or not) can pop up a fraudulent credit card payment interface at any time.