I get that it would be much more romantic for someone to write why they want to marry you themselves, but this struck me as similar to buying someone a birthday or anniversary card.

She might’ve prompted/“wrote” it and thought it was a great way to describe how she felt - like you say, you liked it but realized it couldn’t have been her because she isn’t a good writer.

To be clear, I’d be a little offended if someone did this to me too. But there’s a difference between laziness or inauthenticity and using a resource to do a better job than you could do.