To offer a simple answer: because gardening is an investment, and people prefer to make investments in things they own. To make a broader observation: arguably

To offer a simple answer: because gardening is an investment, and people prefer to make investments in things they own. To make a broader observation: arguably one of the keys to a wealthy society is strong laws that protect private property (houses, businesses, etc.), because of just this phenomenon. I live in a neighborhood filled with beautiful gardens, and I think each one is a labor of love by the homeowner.