In order for any proofreading tool to be effective, it must take into account the context (i.e. business writing, fiction writing, news reporting) and also the intent of the writer.
Consider the following example. If you are writing a resume, and you want to list your work experience, you may write:
"I have had an experience as a secretary."
This is a common mistake for ESL students. This is grammatically correct, but not for a resume. It means, once upon a time I had an interesting anecdotal experience while I was working as a secretary.
A good proofreading tool should be able to suggest the following correction:
"I have had experience as a secretary."
This means, I have worked as a secretary, and is appropriate for a resume.
In this example, the grammar checker will have to understand the difference between a) the context, b) the writer's intentions, and c) various usages of the definite article. Even though data can be collected, as in a corpus, the final analysis must still be done by human beings. If you can create a tool that can do all this other than the human brain, I will fart monkeys out of my butt.