The dynamic is oddly directly similar to how two leading players will interact in a three-player political* game. They will naturally stabilize the game state in a way that prevents the third player from ever engaging.
This doesn’t happen in four player games really. Transient natural coalitions form in twos that tend to keep everyone in the game. But with three players? There is almost invariably one player who is frozen out.
The barrier-to-entry of joining a market for something like “Search” or “Online Ads” is so high that basically every other entity in that space is just the “third player.” Only a large coalition could join such a space and dislodge one of the existing ones. Only multiple coalitions could join and lead to a broader market.
Of course an external event (e.g., a new technology, a radical demographic shift, regulation, etc…) could also alter this equilibrium.
*political as in interactive, not as in actual politics. I.e., players can mess with eachother and the game isn’t just a race.