By allowing you to manually set the status of your projects, you have more control over how your projects are presented.
For example, let's say one of your targets was to increase revenue by a certain amount. The end of Q1 has arrived, but you haven't achieved the targeted revenue percentage for that quarter. However, there are circumstances that you (and your leadership team) are aware of.
Let's say that your organization had targeted a product launch in Q1 that was supposed to be responsible for a significant portion of Q1 revenues. Unfortunately, the product launch was delayed, but your sales team still managed to get very close to the target revenue. You want to mark your revenue status as on-track, because considering the fact that the product didn't launch this quarter, your sales team did a great job, and will undoubtedly make up the shortfall. You can provide this context in the comment section of your update.
This is just one example of why there isn't a standardized status calculation that encompasses every initiative, every department, across every organization.