Why do we have to use two-dimensional lines when applying for a patent for utility model?

Patent documents, if gray-scale views and color pictures are used, are easy to become blurred and patent features are no longer obvious in the process of repeated printing and copying, which is not conducive to patent application, transfer and subsequent patent protection. However, two-dimensional line drawing is concise, and the probability of distortion is small when printing and copying, so it is widely used in European and American patent copywriting.