The problem is that grammar flames are socially awkward. They've got a bad reputation on the web that goes way back to Usenet days. To issue one is kind of like telling someone that they've got some food on their face. But it's worse, because on the web you can't whisper: By default your criticism is preserved forever, smack in the middle of what's supposed to be a conversation about something other than grammar. Unless the author takes the trouble to delete some comments -- which is an extra step, and which can also sometimes be socially awkward, since comment deletion is generally reserved for trolls.

I wouldn't have bothered correcting this one if I hadn't been able to make a lame joke at the same time.

I agree that a feature to fix this might well not have enough utility to be worthwhile, which is perhaps why nobody's built it yet!