Also, "cloning" is a poor word for such activity. Writing code is hard and time consuming even if you're merely replicating functionality of an existing product. To me "cloning" implies that it somehow cheap or easy and it just isn't.
Not all the time to be honest. Creating a finished, complete and useful product can involve a huge amount of prototyping and several redesigns. You have to try lots of ideas that don't work, throw code away, redesign the UI as it becomes more clear how the product is used, test on users etc. If you heavily base your product on an existing complex one you're likely taking a big shortcut.