Detroit is a top 10 defensive team.
Problem is, they're 21st in Offensive Efficiency.  They are 26th in TS%.
Notably, they are 29th in 3P attempts per game and 24th in 3P%.
Compounding this, they are also 29th in FTA per game.
Basically, they're a horrid offensive team.  Can't shoot, and can't generate efficient looks.  As a result they are tied for DEAD LAST in Points Per Shot.
I think this goes beyond simply not having a top scorer.  They just don't have enough efficient offensive players, period.
At first blush they look kind of like a poor man's version of SVG's Orlando teams with Dwight.  Unfortunately, across the board they are worse offensively than those teams.  Drummond is not a very good finisher for such a huge, athletic big.  So they lack that centerpiece on offense.  And the shooters surrounding that big are significantly worse, and more skewed toward taking mid-rangers.