Why do you have to be in better shape to reenlist?

What? This is really vague. I'll try- !. A recruit will be brought to standards. 2. A current soldier must pass the standards and many unit expect them to exceed in minimums and go for the maximum- If you are a current soldier and want to stay in you should be in/and keep in the Army's standards- we don't have time/money to get someone back in shape, and being out of shape means they were not a quality soldier in the 1st place- *my opinion)

The standards are much higher; when one reenlists.

Better shape then what? You have to meet minimal standards to reenlist. If you are talking about compared to a civilian off the street going for first initial enlistment, that is a different situation. The Army realizes that it take an effort to bring a civilian to a level of fitness that allows them to succeed in the profession. There used to be a reduced standard to graduate Basic (and I think AIT) of scoring 50 points in each event. This was in recognition of the time it takes for a recruit coming off the streets to reach an average fitness level. This is changing. Now new (and I believe prior service) candidates need to go through the OPAT (Occupational Physical Assessment Test). This was instituted after gaining units discovered that they were experiencing loss training time and injuries to Soldiers who were not physically qualified for their MOS. Even more recent changes include the revamping of basic training requirements, including raising the APFT to the standard 60 points/event.
This is a response to the feedback units are giving about new Soldiers not having the physical (and often mental) discipline they expected.

Bottom line: the military is expected to perform at a greater level, across the board. Physical fitness standards are going to continue to get a greater focus. If you are so worried about not being able to meet the same standards you were expected to adhere to when you were in before, then you may want to reconsider reenlisting.