I think there's several things going on. 1. Doctors often either don't have time or don't take the time to really review the records their sent. Specialist often have very tight schedules so they don't spend a lot of time preparing for each appointment.
2. Since developmental delays are diagnosed based on symptoms, a lot of doctors want to hear the symptoms so they can draw some of their own conclusions instead of just blindly going by what someone else concluded. It's a little annoying to go through it all time and again but it's due diligence on the doctors part and ultimately means better care for your child.