Why isn't it? There are various package tours operated from LV to the Canyon which include a 40 minute flight, 2-3 hours of tours to various viewpoints and a flight back. It is by far the best option if you are short on time.
A week isn't very long to do what you want to do even without going to the Canyon, if you are going to use up the best part of 2 days of that week driving to the Grand Canyon and back you've even less time. You can't fit in Vegas to San Fran to LA back to Vegas and the national parks into a week anyway - it's simply not enough time. Generally people would spend 2 weeks doing that rather than 1.