He scores quite a lot of goals, but those goals tend to be distributed across a relatively small number of games. Consistency is the issue. For example, this season in the Premier League Lukaku failed to score in 19 of the 36 games he played more than an hour in.
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...
Lukaku is not really the man you rely on to get you a goal a game week-in-week-out, he's the man to pile in on a thrashing and rack up a big tally there.