I'm with Harlequin. I think singing at a professional level requires a basic talent which is then developed, while writing competence can be a learned skill. People can (and those who work at it do) master sentence construction, word choice, punctuation, grammar, and all that goes into basic writing mechanics.

It's worth noting that one's education, reading habits, and openness to feedback figure in. People who received a poor education, for whatever reason, start at a huge disadvantage. Often they do not realize they don't yet write with competence, and learning they aren't as good as they believed themselves to be is a substantial blow. Not everybody recovers. Many don't have the time and will to do the hard work of catching up. It takes some serious dedication to the craft to master written English if you didn't arrive at adulthood with that already in place.

I admire the hell out of people who start from such a level and continue to take criticism to heart and improve over the course of years.

Maryn, lucky in education