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.
My argument is that the point is whether its advantageous to allow companies to continue to grow because whether a company has a monopoly or anticompetitive edge is the central argument behind breaking them up. By allowing companies to become monopolies or near monopolies, you disturb the very unified domestic market that you initially mentioned, which hamstrings growth in the future.
I believe companies aren't broken up because they are now so big, it is a logistical nightmare to do so, therefore those companies lobby the crap out of politicians to kick the can down the road. NVIDIA is nearly 3x the market cap of the next non-US firm...is that really the global competition you're looking for?
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.
[0]: https://companiesmarketcap.com/