I’m sure there are a wide range of reasons and in some cases no reason at all. We’ve got 1 small, 1 medium 2 large outside dogs with a very large yard (could easily build four houses but have main house and 1 bedroom guest house. (In laws).
The outside dogs eat, sleep, bark at strangers .. yard is enclosed block brick. usually two go to the farm with Mama and sometimes Papa. I think the in-laws look at them as to watch the house.
Have 2 little pom pom chihuahua mixed house dogs… one just lays around sleeps and goes outside.
The other is the Queen dog.., boss of all dogs.. very intelligent and fun to be around….. really great
dog.