I always feel that the most difficult part of any project is its design, the implementation is rarely difficult. By design I mean both the conceptual design (data model, user flow), and user interface design. It takes a lot of time to come up with ideas how to tackle a problem. Then you implement them, realise it doesn't work, and start over with a different approach.

All this design work isn't necessary when you copy something. Someone else has already done the design work for you. You can lean back and say: "The solution is obvious." Then you only need to implement it. In my experience, the implementation always seems trivial in comparison to the design process.

But you are right, people who copy stuff often make poor copies. I think the problem is that they probably don't fully understand your design. They see how you solved something, but they don't understand why you solved it that way.

Finally, I don't think a rebuild is the same as a copy, because you generally only rebuild when something doesn't work. And then you actually need to put a lot of design work into the rebuild process -- that's why it's hard.