The reason is that there is profit to be made by inefficiency.

When rail is used, the people who profit are;

• the sender
• the recipient
• the railway company
• the train drivers
• the community.

When Trucks are used, the people who profit are;

• the petrochemical companies
• politicians who receive contributions from Petrochemical companies
• vehicle manufacturers
• vehicle maintenance providers
• vehicle parts providers
• the trucking company
• politicians who receive contributions from trucking magnates.
• road maintenance and construction companies.
• politicians who receive contributions from the construction industry.

These are all at the expense of;

• the sender
• the recipient
• the community
• other road users
• emergency services
• the poor truck driver who is high-as-kite on amphetamine in order to meet his outrageous deadline.

The reason road trains exist is to make trucks more efficient than normal trucks by transporting in bulk - instead of carrying the weight of multiple trucks + cargo, it's only one big truck. More efficient, and therefore cheaper.

Rail is even more efficient than road trains at transporting in bulk from region to region, as they don't waste energy on friction, and are even better at carrying in bulk. Road trains don't exist in most countries because they usually have rail to take its place, trucks have their niche in the first and last legs within small regions. After all, moving a truck across regions is less efficient (and therefore more expensive to do).

The only form of transport more efficient than freight trains are cargo ships, which can't go inland. There's a reason mining companies often build their own freight railways to transport between mine and port.

Australia is finishing a build of Inland Rail, a rail freight corridor right now. I'd hardly call it dead when we are expanding the rail network. Admittedly the initial build is over budget, but the initial build is always the most expensive part.