"Private Property" signs...Common Sense or Law 101 for the Stupid?????

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Okay, so I understand needing the signs for "Private Property" and "No Tresspassing" signs along areas where it's nothing but woods. Some people might not think it's claimed (like what piece of property isn't already owned by someone?). But on your own house? On your own garage? Don't get me wrong, I am not making fun of the people who have to post these signs. I am sure they have their reasons. But isn't it common sense? Why would the law ask, "Well, did you have a private property sign up or a no tresspassing sign up?" Why should someone have to do this. If I look at a house it is common sense to me that it is owned by someone therefore you don't have the right to walk around it, play in their yard, park your car in their driveway. Why does it seem that idiots can get away with crap just because someone didn't have a sign up. Why does a homeowner have to place such a sign in order to keep neighborhood trash off their lawn? It just blows my mind. Any thoughts??