Towing capacity is not just a function of engine horsepower, but of a large group of factors, such as engine, transmission, differential, vehicle frame, suspension, and chassis strength, engine and transmission cooling needs, and a bunch more.

Toyota Marketing designed the Tundra to provide a maximum towing capacity that would be competitive in the marketplace at the time of design.

But the market is constantly changing, and Toyota has decided to continue the Tundra but not necessarily go into the heavy-hauler market.

It may be interesting to note that the great majority of consumer towing is done with loads of less than 5,500 pounds (according to National Highway Traffic Safety Administration data). Trips to a local building supplies store for some lumber or roofing, trips to recycling facilities, moving household effects, small business towing, and limited towing of moderate-weight RV's are examples of consumer towing most frequently encountered.

Because of this, the Tundra has far more towing capacity than the great majority of real-world consumers would normally require.

Should you require a towing capacity beyond the maximum capacity of the currently available Tundra models, it would seem you should look into much heavier duty trucks designed for much heavier towing.