
Because of her naivety with pregnancy and motherhood. She doesn't want to abort but she doesn't exactly want to raise the baby either. Her whole deciding that she wants to keep the child is because of her own prejudice surrounding abortion and the fact that it doesn't feel right to unalive that heartbeat. I have in no way of knowing what a pregnant woman would feel like much less a teen one but the whole hopeful idealistic stance that they both are pursuing will be the end of them. Dropping out of high school and turning down an academic opportunity just to raise a child you aren't even sure you want to raise is just preposterous and I don't think she even understands the whole gravity of the situation yet.
And this is why she's a child