iOS actually does have a separate "internet access" permission for apps, but it's only available for devices sold in China.

It's funny you mention it; I've been cobbling together this link collection the past few days and your comment has spurred me to finally publish it:

iOS: Disable WiFi (not just cellular) for specific apps without jailbreaking https://tinyapps.org/blog/202209100700_ios-disable-wifi-per-...

So it's not the case that it'd be too much work or technically infeasible for such a permission to be available to us. It's just that Apple doesn't actually care about us or our privacy.

Probably impacts ads too much