Yes, Drogba and Rooney weren't very good goal scorers but they were great players so their goal scoring record was good enough as they offered lot more to the team than just goals. They are capable of producing man of the match performance without even scoring a goal, that's how good their contribution was.
You are just proving my wiki point with every post. And no, all this is not to berate Lukaku, it's because that's how people see it. There are players who are great players but not great goal scorers.
Drogba scored more than 20 league goals just twice and after that his best goal scoring season was 12 goals. In 9 seasons, he scored more than 10 goals only 5 times.
Lukaku's contribution is nowhere good enough to carry his goal scoring record while Rooney and Drogba's contribution is miles better, so that their goal scoring record wasn't a problem. Apart form all this, Rooney was not even a lead striker for the club. He played as 'lead the line' striker once in his career, 2009-10, when he scored 27 league goals in 34 games.
If you just go by career totals, then Giggs is better goal scorer than RVN, which I'm sure none of the posters here would agree with.