Because this is SciFi. Is your question serious, or are you joking?
I believe that with this attitide, your answer could be given to a large percentage of the questions here at the TrekBBS!! I´m sure most of us (including the OP) is aware of the fact, that "it is SciFi" (see his post #1). The intent of this thread was (correct me if I´m wrong anyone) to find an in-universe
explanation, as to why Starfleet would go to such trouble to completely rework the Enterprise
instead of just building a new one.