Because it is a complex business model. Well actually two. Freight railroads work by transporting a load of freight from one location to another one far away. Massive long trains can easily transport as much as a 100 trucks. So scale makes it worth it, but for a lot of cargo it is truck -> train -> truck, which means you need a certain distance to make it significantly shorter not to go by truck directly. So a freight railroad needs to build a long railroad to make it worth it and that is a huge initial investment.

For passengers you can see the model in Japan. Basically it is a real estate company improving its land value by offering a better rail connection.

Both require a lot of money.