Doctors sign documents so often that it would require too much time and effort to make every signature perfect. Physicians typically have demanding jobs that are quite time-intensive. Perfecting quick signatures is one of many ways in which professionals like them maximize time spent at work.
Though doctors are known for poor handwriting, this doesn't interfere with their work. In fact, legible signatures become less and less critical each passing year. Over time, digital verification methods have become more relevant and may eventually replace traditional signatures altogether. In the meantime, there are plenty of methods for verifying the authenticity of any doctor's signature. Calling the doctor's office is typically enough to verify any doctor's note or prescription.