Web sites optimized for mobile weren't really a thing when the iPhone launched. That's why apps shined on smart phones. Web designers did eventually catch up with the <meta-viewport> hack but the trend towards apps was already too established at that point and people thought that you need an app to use a service.
Funnily enough, when the iPhone launched, it didn't have a native SDK. With iPhone OS 1, developers were supposed to create web apps. That only got changed with iPhone OS 2.
You are right. Steve Jobs also used the opportunity to destroy Flash. But my point still stands: Most web sites weren't developed with the mobile form-factor in mind until a while later.
Eh it's partially true - the browsers on phones needed to get better as well.
Zuckerberg famously made the Facebook apps basically just a responsive webpage under the belief that maintaining one codebase for all platforms would be best.
The performance on mobile was so terrible that Instagram started eating his lunch simply by maintaining native mobile apps that are actually performant, and Zuckerberg was forced to panic purchase Instagram (made by a team of 13 people) for a billion dollars.
Eventually phones got more powerful and mobile web browsers got good enough that PWAs could perform comparably to native apps, but it was too late, and Apple and Google had absolutely zero interest in promoting something that would damage their app store / play store revenue.
