Why do we ship by truck?

And send cargo by ship?

Because trucks generally can't float on water.

Yeah, and ships can't travel on land.

Because the verb "to ship" originally meant to send something by ship, but today it is figurative. The relation to an actual "ship" (substantive) is opaque.

Same with the word "to arrive" (< "ad ripa" = "to shore"). To arrive originally meant to come to shore (in a ship), but no more.