Because your result only reflects the genetic component. Health conditions and traits also depend on environmental and lifestyle factors, which are not analysed and may play an even greater role. There may also be relevant variants that are not yet known or are not included in the analysis.
A low or average result only means that the variants analysed and included in our model do not place your score among the highest; it does not prevent you from developing the condition. Some people also have hereditary forms caused by rare variants with a large effect, which this estimate does not capture. If you already have the condition, your doctor should assess your case.