It's likely they are making new characters for new storylines. It's also possible that a host was completely destroyed during a storyline (such as burned to ash or blown up) and required a complete rebuild.

There are generations of hosts. There are some that have been present since the park opened. These old hosts are more glitchy, robotic and mechanical in nature. The current generation of hosts are made of synthetic blood and bone. More cyborgs than robots. It is likely when a host gets upgraded, they simply upload their personalities in to the new model. It's stated that these new hosts are cheaper to repair and manufacture compared to the older, robotic types.