You're right to raise apps as a threat to the open web. And about performance being a factor (WASM could help there).
However, the real reason that the ad money (and engineering effort) is going to apps is that app platforms do not protect privacy at the same level that the open web does. There's so much more tracking available on mobile platforms than there is on the open web.
I don't think I agree. The privacy protections are different between web and native apps. On the web, you can track users passively with cookies and with various fingerprinting techniques, and there are numerous ways for third parties to communicate to share tracking info. In native apps, you can use the "advertising ID" (IDFA for iOS, AdvertisingId for Android) the advertising ID is designed to change from time to time or even restricted by savvy privacy-sensitive users, and communication between third party apps is more restrictive. Privacy is mostly a fiasco in both cases, IMO.
However, the real reason that the ad money (and engineering effort) is going to apps is that app platforms do not protect privacy at the same level that the open web does. There's so much more tracking available on mobile platforms than there is on the open web.