Why didn't they just give the scroll to Tai Lung in Kung Fu Panda? Being the person Tai Lung was he would never have discovered its secret anyway and it would have avoided a lot of misery.
Because by doing that Master Oogway would be recognizing Tai Lung as the Dragon Warrior, in Dragon Scroll we see:
The Dragon Scroll is a legendary scroll, written by Oogway that contains the secret to "limitless power". Oogway declared that only the Dragon Warrior would be worthy enough to read the Dragon Scroll. It rested for nearly a thousand years at the back of the Hall of Warriors just above the Moon Pool in the mouth of a golden dragon statue, until the events of the first film when the Dragon Warrior (Po) was finally given the scroll.
In another part:
Unfortunately, Oogway saw darkness in Tai Lung's heart, and refused him the title, as well as the Dragon Scroll, which was being kept inside the Hall of Warriors.
After those events, we see in Dragon Warrior that the Dragon Warrior must be someone who fights against the evil:
When Master Oogway received a vision of Tai Lung's return, he declared it was time to choose who will be the Dragon Warrior, the one who would stop Tai Lung and save the Valley of Peace.
The role of the Dragon Warrior is much more than to be a simple fighter, in the Kung Fu Panda 3 section in Dragon Warrior we see:
In this movie, it is ultimately revealed that the true and ultimate role of the Dragon Warrior is to be the one that will revolutionize the world of Kung Fu and lead it to a new and better era, a potential Oogway saw in Po due to him being a panda, a race who taught him the ways of chi, and therefore could unite both the secret race of pandas and the rest of the Kung Fu World. Also, it is his role to become a true chi master and defeat Kai.