The simple fact is that you feel way more secure in your house than you should. Something as simple as locking the door gives you that satisfying sense of being safe and secluded. Unfortunately, it is a completely unfounded feeling.
Your front door is the easiest and most likely place of entry, before the undead horde rises, during the looting while society falls apart, and afterwards when a mass of flesh is pressing against it in a senseless drive to get to your oh so tasty flesh.
The average door is just not designed to take any real force, especially when all the pressure if focused on the strike plates where the bolts from the locks slide into the frame of the door jamb. Many law enforcement and security articles will tell you that it is surprisingly easy to kick down a door. The jamb around the bolts is always the point of failure as it tears right through the wood of the frame.
Throw a few undead bodies at your door and let the pressure build up behind them as they hear you scrabbling about inside, and you will have the same effect.
Not to worry though, the white metalic object in the image below is going to buy you the time you need to grab your bug out bag and make your escape out the back. With the door jamb armor in place the force of any impact is distributed across a much wider area, and when properly installed (in combination with a high quality door) can provide you the time you need to survive.