I think it's because, past autocomplete, for AI to be useful professionally you need to already have a lot of background and experience in what you are using it for, in addition to engineering and project management to keep the scope on track. While demos with agents are impressive in practice autonomy is not there they need strong guidance, so it only works as very smart assistant. What you are describing is very representative of this.
If you don't have that level of seniority then you'll struggle to get value from AI because it'll be hard to guide and keep on track, also spotting and navigating errors and wrong thinking paths. You cannot use it as an assistant, only takes what it says at face value, and given it'll randomly be wrong it makes it useless.