Pretty much the same reason the entire "beast" type of 5E is dramatically weaker than other types at the same CRs. Thanks druids =)Eagles, hawks and snakes are prominent examples in the book. The strongest critters are weak pack critters, not bruisers.
They contribute to combat, in a way comparable to the equivalent level features of other subclasses as demonstrated above by @Sacrosanct . They are bonuses and tactical options for the PC, not agents in and of themselves.
Edit - Whoops - Bring back skill challenges in a big way! Front and center!