Because a diphthong is fine. Taking a already used character and assigning a new sound to it is going to make things hard.
Also I need you to argue not just from the English point of view, but all Latin alphabet using languages, in particular those with strict rules of pronunciation like German.
I think you really miss the point. It’s as if your suggestion that romanization methods have imperfections dismisses the actual reasons why people will refuse to make the effort to learn how to pronounce a name from a language other than their own, which go far beyond whether or not the spelling “makes sense”.
The comic gives a very concrete example of that. It wouldn’t matter if the letters exactly mapped to a perfect pronunciation, the mere fact it does not roll of the tongue, i.e. “sounds foreign”, coupled with the underlying xenophobia+racism combo is what’s at work there.