Google takes all of AdWords, since it's on their own site. They take a 32% cut of AdSense.

The thing is, advertisers could use other channels to put ads up on the web. They can advertise with Chitika or whatever and get a bigger cut than they would with Google. They typically don't, because AdSense adds a lot of value by targeting the ads well. On the other hand, if you want to sell software on an iOS device, the only way is through the app store. Apple doesn't need to add any value to protect their 30% cut from competition because they don't allow competitors.