Because she isn't trustworthy! Whether for reasons of immaturity, thinking we are foolish, or impulsivity, she just isn't.
There are some things we DO trust her with, even though she doesn't always get them right. We can send her to the shops for milk, fruit, and a tin of beans and she will go to the shop, spend they money we've given her on things for the house, but forget that we said fruit and instead buy cheese, and even if told not to, she will also buy herself some sweets. She can be told 1000 times that the snacks in the cupboard are for her to take one to school each day next week, she'll say yes, and then eat 4 of them over the weekend and complain on Tuesday there are none left.
She'll tell her flute teacher she was late for her lesson because she was late out of school, when we know she was late because she got home and decided to start doing something else instead of heading off for flute.
She goes "to bed" and puts on her pyjamas but then we come up later and discover she's been merrily emptying my jewellery box in our bedroom or playing with her toys in the study instead of doing anything towards trying to go to sleep.
She tells me she's brushed her hair when I can see it's a bird's nest.
Obviously she'll get better as she gets older, but she can't be trusted with quite a lot of really important things. Even if there are 15 year olds who can be trusted not to scroll all night on their phones, I very much doubt she will be one of them. So we'll increase our trust in other areas, like we are doing.
@NormaMajors1992coat she doesn't have anywhere to post nudes, thankfully, though I don't think she'd take them, she takes loads of selfies, and she doesn't have any social media to post them on. She could be emailing them to people but she isn't doing that, at least.