It's cheaper elsewhere because those countries have set maximum prices. For instance, in Canada, the Patented Medicines Review Board sets maximum prices for pharmaceuticals.
I don't have any kind of philosophical opposition to importing medicines from Canada, but Canada has a practical objection: if we allowed these imports, it's very likely to cause drug import disruptions in Canada. Thus, Canada has asked the US not to do this. [1]
I think this is very likely a legitimate concern. The US is a much larger market, and a _much_ more lucrative market than Canada, since the US does not fix prices. If the US allowed imports from Canada, it's likely the pharmaceutical industry would stop selling to Canada at the rates set by the PMPRB. The industry would sooner miss-out on 100% of sales to Canada than have their pricing power in the US eroded.
Other than the anomaly of Covid vaccines, Pfizer actually has larger sales in the US than the entire rest of the world put together, which tells you something about how much the US is overpaying.
If the US wants to set maximum prices for drugs, and we have the votes to do that via importation, why in the world can't we just have our own Patented Medicines Review Board, rather than relying on Canada to set policy for the US?
[1] https://www.reuters.com/article/us-canada-pharmaceuticals-ex...