Because it's fun - and potentially dangerous. Either take the trash out each night before bed, or invest in a sturdy can with a lid. Never ever throw things that can be deadly to cats in it - cooked bones, spoiled food, and so on. We had to go with a sturdy can with a lid, and a lever at the bottom that opens the lid when you step on it. It's heavy enough that it cannot be tipped over. But we do remove any 'interesting' waste from the house each dinner time - this way the trash is a lot less tempting. We put it into a grocery store bag, and all litter is scooped into it - this happens 2-3 times a day. Between that and the heavy can, we have no issues any more.