Because it is far cheaper than building a new one, and the production doesn't see filming miniatures as historical assets most of the time. For Star Trek Generations, the original plan was to modify the Excelsior for the Enterprise B, and the designs (by John Eaves, as I recall) reflect that intention. At some point, it was decided to create an entirely new miniature, and we now have 2 separate ships to appreciate. And each of those ships were modified for the various Star Trek TV shows in production during that time.