That Darth is a title, not a name. I mean, why the hell not? First of all, it’s a solid portmanteau (“Dark Lord of the Sith”). Second of all, maybe this is just me, but I think Darth is kind of a funny name, but works fine as a title. Also, despite what people say, there is nothing in the OT that would suggest this shouldn’t be the case.”
I have always considered that “Lord” is the title, hence “Lord Vader”. To me it seems “Darth” is used in the tradition of the Asian way of naming - placing the family name/surname first, followed by the christian name after it.
The best example I can give is with Jackie Chan who was born “Chan Kong-sang” and his father “Chan Chi-Peng”. When Jackie Chan trained in martial arts, under master Yu Jim-yuen, he became known as “Yuen Lung Chen”.