This is my experience too. For any of my day job projects. Which is the best explanation to your title question

Yeah this is exactly it, you might think you are close but you actually just started, and the actual work is still to come.

Whether you want to push through the initial honeymoon phase, where you are done with the fun part that you started the project for, and do all the rest around it that makes it an actual 'product', is up to you.