Earlier quoted context omitted.
In the case of Goldman it's because they do better work than other firms on behalf of their investment banking clients and keep getting rehired, which is why they've had the world's strongest IB franchise for several decades
Unless you count Goldman taking the other side of those same IB deals against their own clients...
While Birnbaum and the other individuals involved in Abacus acted especially douchey in front of Congress, the point that was apparently lost on many people was that when a market maker sells something off their prop books, they are inherently short that thing