Because one guy is carrying majority of the load and makes his linemates have career years, regardless of who he's playing with.
Of course McDavid gets a bit of a bump too, but McDavid is easily the driving factor in Edmonton and he is responsible for most of their plays. There's not a single player in the NHL who has as much of an influence as him.
The biggest reason is the absurd powerplay opportunities and percentage of the Oilers. I've outlined it several times in different threads, but they get an insane amount of PP chances, largely because of McDavid. And the PP is highly efficient, largely because of McDavid. Now we remove McDavid from the team, what happens to the PP ? It takes a massive hit, thus dropping Draisaitls points.
Look at Draisaitls even strength scoring, over the last 4 years Marner has a better even strength goal per game statistic than him. He's an elite player that is a beneficiary of an insane PP being carried by McDavid. Not saying he isn't elite, but he gets his numbers inflated pretty significantly because of McDavid.
There is only 1 McDavid in the league, there are several guys who can be compared to Draisaitl and even better than Draisaitl IMO.