I have a DB dictionary, just skimmed through the section about humans, their culture and all, and I’ve not seen an explanation for this. I will read it thoroughly later, but here are some interesting points to consider, which may, or not relate to that.
The whole world, on earth, although divided by “provinces”, is basically one nation, ruled by one king. I remember that from the sections in the manga when Kakarot was a kid. As well, the Christian god is stated to not exist in this universe. (although there is a “god”, who, as far as I recall only Kakarot and some of his friends met directly)
No Christian god, one nation, could play on last names. Personally I do not really believe, in this world, that relationships have much of anything to do with last names, or their lack.
Also, racial discrimination does not exist in this world. Racism is not a thing. Not sure if that can be related. (although the Saiyans do perceive themselves as superior)