OP, your logic is flawed.
Are you saying we don't seem to have a player that can score 20-30 at will? Or are you saying that the players we have are not capable of scoring 20-30 on any given day.
If the former, then yes. We don't have LeBron, Kobe, Durant and other super stars on this team.
If the latter, then you are wrong.
Jeff Green and Rajon Rondo could drop 20-30 points at will. Pierce can still do it if he wants to. Didn't he drop 40+ points this season in one game?
If someone can motivate Jeff Green and Rajon Rondo to give their best, then they will easily go out there and shred the defense.
For Jeff Green, that motivation is a good team or a "Big Game" against a big team. We have seen him go crazy against Heat and Thunder. In fact, if I remember correctly, he had a breakout game against the Thunder earlier this season.
For Rajon Rondo, that motivation is the playoffs or much hyped-anticipated game. For example, the game against the Knicks last year where he destroyed Jeremy Lin and shredded all the "linsanity" garbage. We already know that he steps up his game big time in the playoffs.
So we do have players that can drop 20-30 on any given night.
The problem is the scoring droughts, which in turn discourage us and weaken our defense.