In the accepted answer to the question What would have happened if the Horus Heresy hadn't happened in WarHammer 40k, it is explained that the projects of the Emperor would have been completed, that is mankind would have conquered most of the galaxy, spreading atheism and annihilating the existence of uncontroled psykers. Cutting the connection between the Chaos Gods and human beings would have extremely reduced their forces, to the point that they wouldn't be a threat for thousands years. The time for the Emperor to control the Eldar Webway to safely travel through outside of the Warp influence.

At the same time, the goal of the Cabal is to destroy the Chaos Gods. Their plan is to help Horus to win over the Emperor (by pushing the Alpha Legion to join Horus), since they foresee that in the aftermaths, renegade Legion will turn against each other and lead he destruction of mankind, and subsequently destruction of the Chaos Gods.

The success of the plan of the Cabal was far from certain (it actually failed). They acknowledged it when meeting Alpharius/Omegon to try to convince him to join Horus.

Is it explained somewhere why the Cabal didn't try to prevent Horus Heresy and help the Emperor in his plans? They just didn't see the Heresy coming?