It isn't, for at least two main reasons:

1) In LLMs, every token has some degree of influence on the output. Starting the prompt with "You" and writing it in second person attracts the model towards specific volumes in the latent space. This can have good or bad impact on the output, depending on the model.

2) Instruct-type models are fine-tuned to respond to second-person prompts. "You"-prompts are what those models expect. If you're working with a model that isn't instruction-tuned, use whatever you want.