Great question! Complements in capital markets typically aren't as strong as other markets like event tickets. On the complements side, hedges are a good example. Market makers like banks are willing to quote much larger sizes for hedged transactions, e.g., an institution that wants to buy a large block of equity in one company while selling others with similar qualities as "factor hedges." The net notional changing hands is roughly the sum of the parts. Still, there's a big price difference between doing this trade atomically and as a series of transactions where the market maker has to "wear" the risk for some period.

Substitutes in capital markets are ubiquitous. There might be hundreds of candidate hedges in the example above, but given how trading workflows are, there's no way to communicate that amongst market participants. A market maker has no way of knowing if someone wants to buy SNAP (and thus potentially has market-moving information about it) or if they're using it as a hedge for a short position and would substitute something that the market maker wants to gross down on (and offer a more aggressive price because of that). As such, market making is a game of pricing under risk and uncertainty. Combinatorial auctions eliminate much of the uncertainty.