They can't worry about next year until this one is over. And it can be over so quickly. Anything can happen. You've gotta take care of right now. Injuries can happen.
Players can leave.
Ask Duke, which had three players leave early last year. Ask Dan Marino, who certainly couldn't have envisioned his first Super Bowl would be his last. You don't know who will be back. There are no guarantees you'll have the same team or even the same coaching staff. So you have to live in the moment.