It's not like Don completely can't sing either. I found this video from 2014. He's really singing live here (little imperfections here and there), and he can hit the high notes in "Witchy Woman" and the high part in "Heartache Tonight" in his real voice. I don't know why he feels the need to lip sync almost every performance now.

https://www.youtube.com/watch?v=VzO6KHm2MBk