One reason is that all the uninhabitable land on earth that gets sunlight, already belongs to someone. It is possible to colonize open sea, and some people are

One reason is that all the uninhabitable land on earth that gets sunlight, already belongs to someone. It is possible to colonize open sea, and some people are working on it. But the main reason to colonize other planets is the potential future payoff, whoever gets there first is going to get much more than people who figure out how to irrigate sahara.