I think if you're writing a 2nd POV/self-insert type story I think the variables are good for the reader to customize, much like a Dating Sim game. Or if the characters are less about their name and more about their role (spouse, neighbor, boss, etc.), then having customization can be nice.
But sometimes an author just feels like Ben is Ben and Kate is Kate and it's hard to break that, especially, as stated by others, if nicknames are involved. Or if the name has significance. Fake Example: Stella Comet: Sexy Space Pirate. Making her Tiffany Zakowski, even if that's your crush's name, kind of makes the character lose her magic.
Though I do understand if Ben, Kate, or Stella are your parents names or the name of an ex who did you wrong so reading the name might trigger a negative or uncomfortable association.