Or Zouken could cause said complex problem, then offer to fix it in return for the wish. It's easier if all the variables are under control, after all. Plus, I kind of have trouble imagining Zouken going around looking for a suitable teenaged girl. (I think he probably would reject the idea of using Sakura, as she's integral to his plan with the Holy Grail, and using her would basically be putting all his eggs in one basket.)