Why is catabolism exergonic and anabolism endergonic?

Breaking bonds requires energy, but there is energy contained within the bonds and that is the energy that is released.  That is why reactions speed up with increased temperature.  That energy is the activation energy.  Once the bond is broken, the energy needed to break it, plus any energy contained in the bond itself are released.

In forming bonds, you need to be able have enough energy to generate the high energy state in which the bond broke.  It takes more energy.  Enzymes are crucial because they lower the activation energy for a specific bond.