Hahaha, QFT. Isn't that pretty much word-for-word what Rise Against said? I'm getting really rusty, since I haven't really listened to punk in years, but...gah.
I think the reason for left-leaning musicians has to do with personality types though. Musicians and artistically-inclined people tend to be much more in touch with their feelings than their rational faculties, and that's pretty much the common thread among progressives. Once you throw in identity politics and group-think conformism (ironic in the case of punk), the whole scene is just a breeding ground for progressivism, even among so-called anarchist punks, who have essentially become parodies of themselves. I stumbled across punkvoter.com in the early-mid 2000's, and even then (when I had begun to basically consider myself a progressive-leaning liberal) I was shocked by the overwhelmingly partisan nature of the site. It was as if all it took to be punk and anti-establishment was to vote Democrat. It disturbed me, because even then it was clear to me that the Democrats, as a major party, were a seriously entrenched part of the political establishment.
Nowadays I'm more cynical, so I pretty much just expect musicians to be idiots. That's not to say they aren't verbally talented, or that they aren't clever, but...cleverness is shallow, and it's no substitute for critical thinking. Every once in a while an exception will come along, and I half-wonder, "I must be mistaken. They have to be idiots in reality; I just must not be looking hard enough."