Can't get a train track to every single depot and loading dock in the country that receives shipments (which is like, practically every big box store and warehouse there is). There has to be a handover at some point.

Edit: also not a big fan of the train system in the US, since the vast majority of rail is privately owned. The operators have too much control. They'll charge towns extra to put automated crossing guards on their rail and then keep charging them for its maintenance. The jurisdiction can't use their own third party workers to maintain it. The railroads are legally only required to put up a sign. It's extortion if you ask me.