Because an interest in astronomy transcends the utilitarian need for a job, and there is always hope that you may be one of the lucky ones who does well in the subject as an undergraduate, gets into a good graduate school, wins a PhD and gets a job. I see no harm at all in majoring in a field you love, for the pure enjoyment of learning about it. You should also 'minor' in computer sciences or statistics or a related technical field which will serve you well as an astronomer, and also be a source of income if you don't get into astronomy. All astronomers HAVE to be highly skilled in computer programming, statistics, mathematics, working with data, solving problems. These are also skills very much in demand in the market place.