There are a myriad of reasons why plebe summer staff is changed out in the middle of the summer.
First, the firsties need to complete their 1/C cruise and other summer programs and take leave. If they spent 6+ weeks on plebe summer, there would be time for little else.
Second, it is good for the plebes to have fresh leaders. It provides a change in leadership style and lets those plebes off to a great or terrible start have an opportunity to create new impressions with new leadership.
Third, it allows more 1/C to get this leadership opportunity.
Yes, it is tiring. But that alone would not be reason to switch mid-stream absent the others. Leading troops in battle day after day is certainly more tiring than running a bunch of plebes around.