I used to work as a proofreader for MIRI, and was sometimes given documents with volunteers’ comments to help me out. In most cases, the quality of the comments was poor enough that in the time it took me to review the comments, decide which ones were valid, and apply the changes, I could have just read the whole thing and caught the same errors (or at least an equivalent number thereof) myself.
There’s also the fact that many errors are only such because they’re inconsistent with the overall style. It’s presumably not practical to get all your volunteers to read the Chicago Manual of Style and agree on what gets a hyphen and such before doing anything.
I used to work as a proofreader for MIRI, and was sometimes given documents with volunteers’ comments to help me out. In most cases, the quality of the comments was poor enough that in the time it took me to review the comments, decide which ones were valid, and apply the changes, I could have just read the whole thing and caught the same errors (or at least an equivalent number thereof) myself.
There’s also the fact that many errors are only such because they’re inconsistent with the overall style. It’s presumably not practical to get all your volunteers to read the Chicago Manual of Style and agree on what gets a hyphen and such before doing anything.