I cannot give you proof for the line of political thinking. :)
> ...having a monopoly in one country is a strategic advantage over other countries.
Having a large, unified domestic market is a strategic advantage because it enables companies to grow to a size that makes them formidable global competitors [0]. The United States and China are examples of this phenomenon. The point isn't whether it's advantageous to allow such companies to become monopolies. Once these companies reach a certain size, politicians are reluctant to break them up because they don't want other global companies to take their place.
