> From the 9th century, Swahili merchants on Zanzibar operated as brokers for long-distance traders from both the hinterland and Indian Ocean world. Persian, Indian, and Arab traders frequented Zanzibar to acquire East African goods like gold, ivory, and ambergris and then shipped them overseas to Asia. Similarly, caravan traders from the African Great Lakes and Zambezian Region came to the coast to trade for imported goods, especially Indian cloth. Before the Portuguese arrival, the southern towns of Unguja Ukuu and Kizimkazi and the northern town of Tumbatu were the dominant centres of exchange.
Even the large rivers that do empty into the ocean, they have significant portions of rapids that make it impractical to use as a trading route.
A chunk of it is about the nature of the lack of good harbors along the coast that would be able to shelter trading ships from the open ocean.
Zanzibar is the noted example of a port https://en.wikipedia.org/wiki/Zanzibar#Before_1498
> From the 9th century, Swahili merchants on Zanzibar operated as brokers for long-distance traders from both the hinterland and Indian Ocean world. Persian, Indian, and Arab traders frequented Zanzibar to acquire East African goods like gold, ivory, and ambergris and then shipped them overseas to Asia. Similarly, caravan traders from the African Great Lakes and Zambezian Region came to the coast to trade for imported goods, especially Indian cloth. Before the Portuguese arrival, the southern towns of Unguja Ukuu and Kizimkazi and the northern town of Tumbatu were the dominant centres of exchange.
Even the large rivers that do empty into the ocean, they have significant portions of rapids that make it impractical to use as a trading route.