Oh, but you do! Not all till shortages are due to theft, and not all theft at cash registers happens in ways that the till is off. Folks do lots of tricks to try to make sure the till count is correct: For example, scanning cheap items, not scanning things, charging more than the register total (and pocketing the rest)... and so on. Occasionally, it gets you out of being in trouble (some dude took goods and walked out of a place I worked at, after I scanned it but obviously before he paid, for example). Most places have variance built in as well - no one is perfect, after all. Some are stricter than others, and many places share the till between people because it takes more labor to do otherwise.
The till not being able to catch everything is a reason for the cameras, and the reason they aren't just around the till. (cosmetics often have cameras as do receiving areas). Bag checks are really common as well. Sure, you could just have an inventory management system, but again, these aren't foolproof nor do these systems give you a clue about who is stealing or if it is a simple mistake.