Well, maybe not... but I remembered this age-old complaint after seeing this Guidelive article where critics are forced to answer the most common reader complaints. One of the most common is the idea "those who can't X, do Y" (you know, like how gym teachers are really frustrated athletes). The answer that Mike Daniel is not only a little too highbrow to really address what readers are saying, he's also condescending there (about the readers, he says 'most of whom aren't pros and have never been "there" either'). The best way to address the idea of writers as frustrated musicians is just to look at the experiences of scribes.
It's probably easiest to start close to home. I took piano lessons as a kid and pretty much stopped by the time I got to high school. If I was forced to, I could probably remember "Hungarian Dance #5" and the intro to "Martha My Dear." During college and a little bit afterwards, I played a little guitar, just enough to figure out a few chords and play simple Neil Young songs. And that's about it. If I ever had any burning need to hit a stage or recording studio, that was quickly squashed once I started reading articles and books about some of my favorite performers and how rosy their lives really weren't.
It got even worse when I started interview dozens and then hundreds of musicians and kept finding the same thing, only worse. "What a crappy f-ing life they lead!" I thought. That also made me admire them more for sticking it out regardless. Most of them never tasted any success and most who did found it fleeting. In between, you deal with devious managers, lawyers, labels, promoters, club owners, band mates, etc.. Cram yourself in a van, play dozens of shows, eat junky food, sleep when you can. And what do you have to show for it? Plenty of cautionary stories. Sounds like heaven, eh?
Then I think of other writers I know. They're obsessed with music and writing and some of them also do music on the side but most of all, they're obsessed with their writing. Sometimes it's getting an idea across or ego or chest-thumping or promoting a band they love or hell, it's just making a buck but in any case, it's about doing journalism. Nowadays, it's easy to record any kind of music and put it online so there's no excuse or reason that should stop anyone who wants to. And yet again and again, most of these writers I know just want to... write. Speaking with them, they love to gab about music and their favorites and trends going on, but I never hear "gee, I wish I could start a band" or "I should put a demo together." They're just not interested in that- if they were, they'd probably be doing it if it was something that was really of interest to them.
I'd guess that most music writers have also seen the unseemly side of the music biz and been dissuaded of even thinking about being musicians but more likely, it's a simpler reason why they ain't musicians- they just don't wanna be. They've already found what they like doing and know that doesn't mean they have to jump to the other side and do music themselves.
There is a history of writers going to music (Patti Smith, Lenny Kaye, Morrissey, Neil Tennant, Chrissie Hynde to name a few) but that doesn't mean the rest of them have the same dreams and ambitions. Believe me, you wouldn't want that- I've heard some scribes sing at parties under the influence and it isn't pretty. My own singing voice has been banned around anyone with a heart condition, just to give you an idea.