I used to complain about this until I had one heroine who did let me pick her name. It was miserable. I spent weeks and weeks looking at all my name books and at a bazillion online naming sites. After that experience, I conceded. Life is easier if I let them tell me who they are.
And yet I still had this conversation with a heroine of mine recently. Maybe I haven't quite learned my lesson.
Me: You can't go with Arizona. I have another heroine named Phoenix and her story is written. We have to change your name. How about Ariel? It's close to Arizona.
Heroine: I'm not a Disney princess.
Me: That can be part of your internal conflict. Your parents wanted a Disney princess and they got a scientist instead.
Heroine: (silence)I take her silence as agreement. I envision how this conflict between who she is and who her parents want her to be influences her life and her choices. I start to make notes, but I can't write Ariel. Every time I write her name it comes out as Arizona.
At first I think it's me, not able to mentally make the change yet. Then I see her expression. My name is Arizona, she says.
I sigh and surrender. Arizona it is.