Good question. Why is that?
More importantly, I have a fence around my yard and keep my doors locked, yet last year my house was still broken into. Why is that?
The fence is around my yard because most people understand its message and respect it. I have a cattle gate across my driveway. When I close the gate, people don't come up my driveway. If somebody wanted to rob my house, it wouldn't stop them. They would just open the gate. But the fence and gate is still effective at protecting my privacy, and it prevents casual theft and vandalism from people wandering through my property (e.g. kids taking a shortcut). It also reduces excuses for people to intrude on my property--e.g. donkey escapes from the neighbor's pen and wanders onto my property meaning neighbor comes onto my property to retrieve it, and yes, that actually happened once before I had the fence.
You seem to be saying that fences are pointless, but they're clearly not. The question should be, "should we build a fence, and if so, how high should it be, and should it be electrified with barb wire, or just white pickets." Arguing that fences are pointless just makes it easy to dismiss you from the argument altogether, and that's not helpful to anybody, yourself included.