why didn't they just do a precision strike, take out Saddam and his crew, and leave?

They tried. For quite some time, attempts were made to specifically target Saddam and his leadership, and take them out surgically. Through luck, and in one case, an informer, Saddam escaped each time.

Had it succeeded, I don't know how much difference it would have made in the long run, especially had we just "left." Saddam's sons, or some other Baathist clique, would have taken the reins and gone back to square one.

The "not enough troops" argument also tends to overlook the fact that more troops didn't exist.