We don't all know that karma is real. I've known plenty of bad people who never got reprimanded, and plenty of good people who never got rewarded. Humans, by nature, love to make associations in their head. For example if someone says "I haven't been sick this year" and forgets to knock on wood, then when they do get sick, they think "this is bad luck because I didn't knock on wood". But if they don't say anything about not getting sick, but they still get sick, they make no association to bad luck, it 'just happened'. In other words, superstition exists because we make it exist. We will naturally find something in our memories to make the connection, whether we want to or not. Weird bad things happen every day, but we only notice them when we didn't knock on wood, or think that karma is going to come back to us. Suddenly your daily trip over that child's toy isn't just a normal occurrence; it's because you walked under that ladder several weeks ago. With billions of people moving about every day, weird things are bound to happen. Some will be explained as miracles, other as karma, but ultimately it's just statistics.