They've been desensitized to human presence just like a park deer. There are park deer that will let you walk up to 15' of them. I've walked up to elk bulls within 40 yards, and my buddy went to 20 yards, without them spooking in Estes Park. I've seen bucks bed down within 40 yards of a barking dog that was chained. The buck learned that the dog would never come after him. It's all conditioning.