My son did the same thing, still does actually. I think for his case it's tied to perfectionism. He doesn't do it until he feels he's good at it. He's actually just now, at 6.5, starting to read aloud and read chapter books, both of which he's been able to do for a couple years but if he didn't know one word he felt he wasn't able to read it. What he does do is read signs and other things out loud, he doesn't think of it the same way.
Try not to sweat it, she will when she's ready and I bet you see her start doing it when she's distracted.
