Why are catabolic reactions exergonic and anabolic reactions endergonic?
In most chemical reactions, including those of catabolism and anabolism, some bonds break and others are formed. For example, if you hydrolize a biopolymer such as a polysaccharide or protein into its building blocks, you break bonds in the polymer and in water, but form new bonds between parts of the water and parts of the polymer.

Making a polymer from building blocks sometimes is endergonic (like some anabolic reactions) and sometimes is exergonic (like making polyethylene from ethylene). There is no inherent rule that says more complex molecules have higher or lower Gibbs free energy than the small molecules they are made from.

You could say there are three reasons why we eat food: To directly use the substances we ingest (e.g. vitamins), to use the substances we ingest to make new substances we need (e.g. amino acids in proteins), and to oxidize substances (in humans with $\ce{O_2}$ as the terminal electron acceptor) and harvest the Gibbs free energy. We choose food accordingly (e.g. we don't eat sand). Also, we don't eat Grignard reagents or other highly reactive substances because they are usually not available and we are not equipped to handle very exothermic reactions in our bodies. So the food we eat is fairly stable, and often it requires Gibbs free energy to drive biosynthetic reactions (explaining why many anabolic reactions are endergonic)

Molecules that can be hydrolyzed in the digestive tract using exergonic reactions are good as food. It is difficult to run endergonic reactions in the digestive tract (the reactants often used to drive endergonic reactions, most notably ATP, are found inside cells, not in the stomach or intestine). For those reasons, any large molecules we digest can be broken down to smaller molecules in exergonic reactions (these smaller molecules then enter cells, where further reactions could be exergonic or endergonic).

Specifically for hydrolysis (breaking biopolymers into building blocks) and condensation reactions (making biopolymers from building blocks), the hydrolysis reactions tend to be exergonic and the condensation reactions tend to be endergonic. This might be related to the high concentration of water throughout the body.