Modern cameras give a way better view of what's behind and to the side of the car than you can ever achieve through mirrors. There's really no comparison. Yes, if you're comparing to older, inferior camera systems mirrors might be better, but not the modern ones.

I say this as someone who learned to drive and drove for a long time with no cameras. People saying this are just usually just used to mirrors and never bothered to take the time to learn to switch to the cameras for backwards maneuvering, or their cars just are not equiped with the latest tech.