Why do decision trees have low accuracy?
It's not true in general. Decision trees tends to overfit in comparison to other algorithms, which provide too low accuracy. But if you use a decision tree in the right way i.e you prepare data in the proper format, use feature selection and perform k-fold cross-validation everything should be ok.

I am sure that you misread it. There is no reason why DT could be a much worse algorithm compared to others.