Defining your own unique project is more fun.
But, wider scope → more diverse projects → harder for us to go deep into your project. The collective know-how we (i.e., students and the teaching team) have built up about SE issues related to the project become shallow and stretched too thinly. It also affects fairness of grading.
That is why a strictly-defined project is more suitable for a first course in SE that focuses on nuts-and-bolts of SE. After learning those fundamentals, in higher level project modules you can focus more on the creative side of software projects without being dragged down by nuts-and-bolts SE issues (because you already know how to deal with them). However, we would like to allow some room for creativity too. That is why we let you build products that are slight variations of a given theme.
Also note: The freedom to do 'anything' is not a necessary condition for creativity. Do not mistake being different for being creative. In fact, the more constrained you are, the more you need creativity to stand out.