... because their offensive line is still their lowest-graded unit on the team per the analytics of Pro Football Focus (PFF).
Joe Burrow currently has PFF's highest grade for this season, among all Bengals players, and he's been helped by his skills of not getting rattled and good pocket awareness to overcome some of the OL shortcomings. Receivers getting quick separation from defenders have helped a lot too.
The Bengals upgraded their defense immensely with free agents, which is bizarre behavior for them because owner Mike Brown has been stubborn about building a team around the drafts. That might've worked out fairly well for his father, Paul Brown, but their draft picks out of college have too often been duds after Paul died.
Yet they're still in very good shape, in terms of salary cap. They just don't lead the NFL in salary cap space this year, like they often do, but they're still near the top in room to spend.
If they can acquire some proven blockers in free agency next year, they should do it. "Joe Cool" has avoided another serious injury so far this season, but their turnaround season will be fruitless if it happens yet again. Seems to me like a question of when it happens, rather than if, unless the Bengals front office takes it more seriously.