A killer app in the like-an-iphone context is something that provides obvious value - if not outright delight - to a huge demographic.
Coding doesn't do that, because the demographic interested in coding is not huge compared to the rest of the population.
Chatbots don't do it either because they're too unreliable. I never know if I'm going to get a recommendation for something the LLM hallucinated and doesn't exist.
There's also huge cultural resistance to AI. The iPhone was perceived as an enabling device. AI is perceived as a noisy, low-reliability, intrusive, immoral, disabling technology that is stealing work from talented people and replacing it with work of much lower quality.
It's debatable how many of those perceptions are accurate, but it's not debatable the perceptions exist.
In fact the way OpenAI, Anthropic, and the others have handled this is a masterclass in self-harming PR. It's been an unqualified cultural disaster.
So any killer app has to overcome that reputational damage. Currently I don't think anything does that in a way that works for the great mass of non-technical non-niche users.
Also - the iPhone was essentially a repackaging exercise. It took the Mac+Phone+Camera+iPod - all familiar concepts - and built them into a single pocket-sized device. The novelty was in the integration and miniaturisation.
AI is not an established technology. It's the poster child for a tech project with amorphous affordances and no clear roadmap in permanent beta. A lot of the resistance comes from its incomprehensibility. Plenty of people are making a lot of money from promises that will likely never materialise.
To most people there is no clear positive perception of what it is, what it does, or what specifically it can do for them - just a worry that it will probably make them redundant, or at least less valuable.