Just because its easy (relative to everything else, also it depends on what you mean by "all the details"), doesn't mean it doesn't take time and therefore money.
You might argue, "well then generating the code is still a win" and it might be, but its a micro-optimization. If the AI can do the easy part but not the hard part, its akin to shaving seconds from an operation that takes hours. Its focusing on the wrong thing. If the AI could instead do the hard part, you would save a lot lot more effort and therefore money. Once that's done, by all means, automate the easy part too, but until then, the priorities are off.
You might still say that this is worth it, saving those seconds still means you don't have to pay for them, even if the majority of the cost is still there. This is possibly true and all well and good. I don't really care, because I'll still have a job doing the hard part. The issue is that when these AI's are mentioned, the "hard part" is always glossed over, the AI is sold as this thing that will automate all development, where it really should be sold as a thing that shaves a little off the total cost of development, but that the hard and therefore expensive parts are still there.
Sure, there are some development tasks that are fairly trivial and some companies that do mainly these might go out of business, but you still have the stories of oracle selling a website to the government agency for $100 million, because it has to interact with a slew of legacy systems, deal with ambiguous tax codes/regulations/requirements. There's a lot of tech out there that has these complexities and that's not going to be automated by these AI's for now, until they tackle "the hard part".
A note on "all the details": if you truly have all the details (refined unambiguous requirements, detailed architecture with all the use cases and edge cases outlined and documented, technology tradeoffs investigated and documented etc etc) then great, it would be quick, easy and cheap to implement then. Unfortunately, what is more common, is that a non-developer will say this and "all the details" really isn't all the details at all and just the tip of the iceberg.