I think there are lots of reasons why people keep their dogs outside. I personally think that most dogs belong inside, but people can come up with lists of excuses....

the dog hair
for protection
he's just a farm dog
doesn't have any manners

and it goes on and on

Of couse, if you brush, you won't have as much hair, a dog can protect just as well inside as well as out (if that's what you want it to do), and a little training will solve the "no manners" part. Some people just don't understand dogs and have never been educated about them.