Ok, so you name two... That rarely did it, and only at the tail end of their careers when they didn't have the speed to cover the outside. Woodson played almost exclusively outside for his first 10 year. How many snaps has Sherman, Revis, Peterson, Norman, Talib, Harris played in the slot over the last 5 years compared to outside. How many snaps did Deon play in the slot vs outside? Perhaps the single most talented CB in history. The answer is VERY few, probably 10-20% of their careers.
You can find the anomaly anywhere. Teams make adjustments to fill game plans. I think Patrick Peterson followed Diggs the last time they met because he was a deep threat from there. But it is such a small percentage of his total number of snaps.
There isn't any argument to be made that slot is harder than the outside. Just from pure talent perspective. OBJ, Evans, Dez, Jones, Jordy... The best of the best, spend their time outside more often than not. Their is no slot receiver in the league that is close to the talent of those 5.