By way of comparison, while Lukaku scored a goal in 47% of the games he featured in, you have players like Aubameyang (71%), Morata (62%), Suarez (65%), Lacazette (71%), Dzeko (59%), Cavani (69%). In fact it's hard to find any other high-scoring striker who fails to score a single goal in more than half the games they play in - Belotti's record is better, Kane's is better, Ibrahimovic's is better...
Would it be possible you looked at it from a 3-year trend point of view, as opposed to just 2016-17?
Also, you can make a case all these strikers you mentioned have had better supporting cast than Lukaku's, but I may be wrong.