> there must be downsides too?

Fewer trucks and freight trains. That affects economic interests, some of them quite powerful. (Maritime shipping is 2 and 3x more than rail and trucking, respectively.)

Also, potentially, cars. The lack of a drive-on drive-off ferry in e.g. the San Francisco Bay Area is in large part due to the inflated costs of American ferrybuilding. I believe when Hawaii tried creating its ocean-class ferries, it paid a multiple of what a foreign ferry provider paid for the same ferry. Because the American one had to be built at one of our obsolete shipyards.