How come I have eye floaters, but nobody else in the family has it?

Not genetic. Floaters most commonly arise from aging changes in the fluids in the back of the eyes. These are individualized and do not run in families. If significant or accompanied by light flashing or a drop in vision, you should see your ophthalmologist as soon as you can.
Trauma. Sometimes facial/ocular trauma such as playing sports can cause the vitreous to liquefy at an earlier age. A careful dilated examination must be done as a baseline to insure that there is no silent retinal pathology that can cause loss of vision or even blindness.
Floaters! Floaters are opacities of blurred "pieces" of vitreous (jelly) in the back portion of the eye (the vitreous cavity). Floaters can also be blood or red blood cells. Floaters are usually a result of the normal aging process of the vitreous. The vitreous detaches (pulls away from the retina, not a retinal detachment), resulting in blurred "pieces" of jelly in the eye floating around.