1. To deter burglars.
2. To avoid the grueling, meticulous process of putting them up each year.
3. Some folks are just full of the Christmas spirit all year.
4. They're pretty. Plus, the house feels oh so beautiful with such lovely accoutrements.
5. They're frozen to the house.
6. Makes it easier to give directions to "the only house on the street decked out with lights."
7. They're lazy.