Just native English isn’t enough
Most proofreaders read your work and correct obvious spelling and grammar errors. They need to determine whether your terminology is correct and will not even notice incorrect word choices.
This is especially important if you are an LLM or a PhD candidate who speaks English as a second language (ESOL). You can only sometimes discern the correct word for a particular legal context since you are unfamiliar with English legal terminology. Even specialist translators sometimes choose the wrong word for a specific legal context because they are translating – they lack the knowledge about the law in English to choose the correct terminology for the proper context.