Silly Hikaru Utada Question About Her Name

[Uemarasan]

This is an extremely silly question, but I'm a silly guy, so here goes nothing:

Well, I've been a Hikaru Utada fan for quite some time now, and I've always either referred to her as Utada Hikaru (when I speak), Hikaru Utada (online), or Hikki (in fan forums), but I've never ever called her just plain Utada. So I'm just wondering why so many fans call her by her last name? I know that's the name she uses in the US (aside from Cubic U), but to me it just sounds weird whenever her fans call her Utada. It's like saying "Hamasaki", "Nakashima", "Hirai", or "Wong". It seems very, uh, high school roll call?

Of course, this could just be me being weird. Again