Some authors budget very unrealistically, perhaps just a couple hundred dollars. (I’m not saying this is necessarily true in your case, but I have encountered it.) In such cases, I find myself wondering whom they are imagining should bridge the gap between what they are comfortable paying and what the editor needs to earn in order to pay expenses and support him- or herself.
Obviously there is a big difference. I actually started with screenplays (with some mild success) partly due to hearing that as long as you have a good story, the grammatical errors won’t be a big deal. Well, that was not true. Maybe if a producer liked the story or I was already established but every contest I entered always came back with feedback about the grammatical errors. Even if I won or placed as a finalist, I always received critique on that. Very little is spelling error, it was more about structure, etc.