TL;DR:

• The ability to match demand and supply (allowing for mutual discoverability) thanks to their ownership of demand is only one of the two advantages of aggregators, the other being the ability to allow counterparts who do not know each other to transact with each other.
• In the markets where the supply of aggregators operate, centralization was caused by the demand using popularity of suppliers as a proxy for their trustworthiness.
• As a consequence of the fact that aggregators can provide the otherwise missing trust element between a customer and a product or service provider who do not know each other, and thus popularity becomes a negligible factor in the users' choice of a provider, the distribution of profits in markets in which the supply of aggregators operate flattens over time, whereas the market at the above layer (the aggregators') centralizes.
• The importance of the trust element opens up the possibility of a new threat to aggregators: whereas their self-reinforcing dominant position on the match-making side is hard to be disrupted, this isn't true for their position on the trust-enablers one. In particular, the share of their profits that come from replacing costs to act in trust-less environments is open for disruption from any new entrant with an innovative trust-minimization technology, such as dry technologies.

(The concept of trust in aggregation theory is not novel; Ben Thompson already wrote in 2015: "Airbnb, for example, deals with vacant rooms; what makes it work is the way it has digitized — and thus commoditized — trust. Uber deals with cars; it has digitized both trust and dispatch."; in this essay, I'm expanding the concept with a focus on the effect of trust on the distribution of profits.)

Aggregation Theory

(If you are familiar with Aggregation Theory, you can skip this and jump to the next part of the essay, "Trust and Centralization")

The digital age saw the rise of aggregators, such as Google, Facebook, AirBnB and Uber.

In his Stratechery, Ben Thompson defines aggregators as companies which possess three characteristics:

• A direct relationship with users,
• Zero marginal costs for serving users,
• Demand-driven Multi-sided Networks with Decreasing Acquisition Costs

For example, in the case of AirBnB:

• Users directly interact with AirBnB's app or website.
• It doesn't cost anything to AirBnB to serve one more user, as it doesn't own nor ship the apartments.
• When AirBnB was small, it was quite expensive to get a new tenant to list his apartment on the website (the founders were going door-to-door!); as it grew, tenants were progressively more incentivized to join the website as that was where most users would go to look for an apartment to rent.

Trust and Centralization

There might be many reasons why many markets become centralized over time: compounding, economies of scale and network effects are the most common. However, these are not determinant in any of the markets where the suppliers of aggregators operate. Here are some examples: