We are a mobile culture. The vast majority of us begin our day by getting into a vehicle and driving to work. Some of us, once we reach work drive again to reach work details. We are accustomed to driving everywhere. We drive to the store for groceries and supplies; we drive to schools, work, doctors, etc.
When the SHTF most of us who live in a densely populated area are not going to be entirely comfortable just hunkering down where we live and hoping for the best. Those that choose to bug-in in a suburban or urban environment are pretty much going to be sitting ducks for the lawlessness that will follow.
You may have a decent supply of firearms and ammunition, fortified defenses and large stocks of provisions, but in reality most every house is susceptible to penetration and destruction. Fire is a prime example. Are your defenses such that if your house was burning down around you that you could bug-in and survive?