How come my mom tells me not to aim too high when it comes to goals?

Ok im 20 and in college and my mom is always telling me not to aim for unrealistic goals in life. Now it's not like i wanna be oprah or anything but i do wanna be really successful and possibly be rich. I wanna go into photography and possibly start my own successful photography business. It really makes me sad cause i feel like she doesn't believe in me :(