You're assuming that understanding it was his priority in the first place. It wasn't.
Imagine a painter saying "these software engineers are profoundly stupid, I spent a whole day explaining to one how different color combinations are differently interpreted by different cultures and he still wouldn't get it".
That happened when Trump was discussing trade issues with the EU. Jean-Claude Juncker, the president of the European Commission at the time, had to prepare colorful flash cards in simple language to explain the issues to Trump, because Trump didn't know at all what he was talking about.
A similar thing happened between him and Merkel, when Trump didn't understand that the EU was a unified trading bloc with a common market and that making separate trade agreements with Germany was not possible. Trump allegedly asked Merkel more than 10 times why he could not do that.
I don't think Americans still understand what a laughing stock their president is to the rest of the world. This goes beyond personal sympathies and antipathies; he is objectively a very unintelligent person and his exceptionally poor manners make things only worse.
[delayed]