Some of us had it drummed into us by our parents "don't answer the door"; admittedly this meant don't answer it as children. But old habits die hard.
But depending on where you live, sometimes more than half the callers are ne'er-do-wells or unwanted callers, such as chuggers, burglars casing the joint, Jehovah's witnesses, salespeople. If you see roads where every other house has a "no cold callers" sign, there's a reason for that.
Business models are based on a certain failure rate: a chugger might expect 10% of people they ask to sign up, a leafleting company might expect 1 in 1000 people to buy from them. This is why they do junk mail and cold calling so ruthlessly; they're expecting most people to refuse, so they try a large number of people.
What I cynically wonder sometimes is if chuggers "share information" about who the "yes-people" are. Officially they probably mustn't because of the data protection act, but I wouldn't be at all surprised if they do. With internet advertising, I very rarely click on a pop-up advert, because I'm sure that doing so adds me to a database of "this person clicks on our ads, keep spamming them!" We all know that if you do sign up to a charity, they see you as a "yes-person", so they'll be calling you to ask for a bigger donation.
So therefore, some of us take the view that a closed front door gives the message to those who persist in the name of business: DON'T EVEN BOTHER. 