Because, one way or another, you're still going to need to become fluent enough in the problem domain & the API that you can fully review the implementation and make sure chatgpt hasn't hallucinated in any weird problems. And chatgpt can't really explain its own work, so if anything seems funny, you're going to have to sleuth it out yourself.

And at that point, it's just 339 lines of code, including imports, comments, and misc formatting. How much time have you really saved?