Yes you can build it in a single prompt, but you miss the 93 versions of iterations and thoughts of the developers who've already made the project. If you have the money, why not buy a product/team that already has a year of experience with real users instead of reinventing the wheel?
That is your assumption. I am not talking about one-shot. I mean that lots are people are going through more iterative development cycles than ever in the history of software. Just my own examples (100% agent written)
https://github.com/brainless/nocodo, https://github.com/brainless/dwata, https://github.com/brainless/akar, https://github.com/brainless/baho
All these and more, from a single engineer with 3 x $20 subscriptions. While I also run chicken and pig farms. It will take time for people outside tech hubs to understand what LLMs enable.