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. They are legally only required to put up a sign.