Like I said – People are only willing to travel a certain amount of minutes. You can’t connect two cities at the opposite ends of the map if the total trip takes more than… I think it is 20 minutes? So you’re forced to either make it a multi-line stretch, or wait for faster trains. (Feel free to correct me)
This time limit is the same reason why people tend to spread their networks early on – While you could definitely keep stacking on more and more trucks/trains on the same Oil Field > Refinery line, you have to be able to ship the goods produced from that amount of production as well, or the Refinery simply cuts down on production (and the Oil Field as well as a result, since nobody will accept the goods down the production line)
Cities are too small to accept huge amounts of goods early on – And cities are often too far away that you can ship to more than two or maybe three cities right away. You might as well spread wide instead of high until those cities you are shipping to grow to a size where they can accept the goods, and allow your industry to grow.