The energy efficiency of a diesel engine is nothing compared to electric power plants.
No, they use diesel motors for almost a century because they are more efficient and cost effective.
As for the rest, that is due to historical reason, back when those networks were established, we didn't have the electrical grid and power available. We do now.
The main reason they still use diesel is because it's cheaper to keep the existing gear running, rather than replacing everything. Electric trains are so much more efficient (in modern kV systems), it's not even funny.