Lots os engineering projects took as long as you mentioned because they were horribly specified. Then the engineer either waits for an answer (which they generally do not get) or takes a decision (which might not be ideal without domain knowledge). Then you release and the customer suddenly starts explaining what they want.

You can improve some parts with LLM (make more prototypes, iterate faster), but if humans are the bottleneck and the problem itself is complex it will not be 10x improvement.

Of course, maybe we talk about different type of project, there are also the type of projects like "a web-shop that sells stuff" that someone considered "expensive to code".