Because you can never have enough!
I thought I had plenty of string cheese from the last time I bought it but it turns out I only had 6 pieces. That is not enough to make cheese manicotti. So I made the sauce, I cooked the noodles, and tomorrow night I will actually bake the dish once I have more string cheese.
Good thing I have that salad to serve as my lunch tomorrow and good thing I don't work tomorrow night so I have time to bake it.
One day I'll learn to plan better.