Why do we continue to transport so many tons of cargo on ships that are too small and lightly built for the load carried?

You mean like the one that got stuck in the Suez Canal?

Which is bigger than any ship in the US Navy

Sorry but it is impossible to make a ship that can carry more cargo then the water it displaces. You are right they will sink!

USN cargo ships are not that big.

International laws define how much freeboard is required for a ship to maintain. One of the factors is the channel depth of the ports of call. Some ports have some very deep channels, others are just barely enough to met international standards. There are even ports that do not met international standards for a sea going channel.

As for shipping on smaller ships- it goes on what ever ship is sailing that has room. With all of  the ships in service, no shipper is willing to wait for a ship that is still under construction. A ship that is still sea worthy will not be retired until it is not sea worthy.

"We" don't.  Who are you talking about?

dunno - i give up why do you

Too small???  Have you seen a megamax container ship???  It DWARFS an aircraft carrier.