Because that's not how the corporate world works. You want to freak out and yell at them? Go ahead. Maybe they don't eat someone else's food for a week. Meanwhile you'll be hunting for a new job. You threaten another employee? You'll be lucky to be employed at the end of the day.
Life is a series of tradeoffs. The squeaky wheel gets the grease. Tim may steal food, but that doesn't mean it's throwing off the status quo. You yell at Tim, you're upsetting the status quo. So go ahead, and have fun with your pink slip. Hope it was worth it.
Or you can keep your lunch at your desk.