Curious seekers that they are, musicians' creative intentions often bleed into other idioms, and then the trouble starts. Even backed by the full might of the publicity machine, few musicians are admired for their literary prowess, thespian skill, or political acumen. For every Leonard Cohen, Kris Kristofferson, or Peter Garrett, there's a thousand Nikki Sixxes, Mariah Careys, or Bonos. The sense of entitlement that follows massive success in a specific field is the most obvious motivation for these multimedia misadventures, but it's also partially a problem of genre: rock & pop are deliberately simplistic & populist forms that often discourage experimentation or analysis. The purposeful, studious effort required to excel in any art form is likely onerous to anyone who just wants to rock.
Sometimes it works the other way: skills cultivated within a certain musical style translate well into other milieus. It's no surprise that as skilled a lyricist as Jay-Z is a decent author & thoughtful commentator; similarly, since hip-hop is all about embodying a persona, MCs often make far more convincing actors (e.g. Ice Cube, Mos Def) than musicians of other genres (e.g. Jack White, Jon Bon Jovi, Sting, etc) - though I imagine anyone who saw Get Rich Or Die Trying or How High would beg to differ.
It may be expecting too much for successful musicians to be skilled in other artistic forms. It's a reasonable assumption that someone chooses a specific mode of communication because it comes the most naturally to them. As the art most directly related to pure sensation, music is an expression of the inarticulable. Perhaps that's why the majority of musicians flex all the verbal dexterity & rapier wit of Koko the gorilla after chugging a handle of cheap Vodka.
I'd prefer artists were at least as lucid as their audience in discussing their art. Unfortunately, there's little in a musician's quotidian routine that would necessarily encourage aesthetic, philosophical, or political inquiry. The internet is an infinitely resonant echo chamber, packed with pop-cultural detritus, from which any musician can hand-pick a grab-bag of references & aesthetic allusions without having to confront said references' original context. Consequently, Lady Gaga can name-drop Andy Warhol & Nietzsche all she likes, but her "smartest" song is barely equivalent to forebear David Bowie's dumbest.
More importantly, the current means of recording have placed an undue emphasis upon the production - as opposed to the composition - of music. As digital technology has made world-class recording tools accessible to the masses, professional recording studios with veteran engineers have become frivolous luxury. So musicians must now craft their own sound-worlds from scratch, unaided, even if they've never so much as plugged in a microphone before. The fashion in which their music is captured & represented is arguably of greater concern than even songwriting.
This, unfortunately, leads to amateurish & underachieving performances of amateurish & underachieving tunes, because it's the only music that withstands amateurish & underachieving recordings. The post-JAMC lo-fi rock 'n' roll revival has been fueled by the fact that it's the only full-band sub-genre that doesn't sound like shit recorded one track at a time using only an SM57. I'd also argue the availability & ease of sequencing & sampling software like Fruity Loops has inculcated the moronic anti-lyricism so prevalent in contemporary hip-hop. Lo-fi, once an obstacle to be surmounted, has become a nostalgic aesthetic retreat for the musically unambitious.
This isn't to say that seasoned engineers make excellent musical directors. Gear-heads are notoriously coarse in their appreciation of aesthetics, since they can find something equally praise-worthy in either Pet Sounds (nice doubling of the piano & accordian!) or Linkin Park (excellent gating on the guitars!). But the musician-engineer relationship provided a valuable & effective division of labour: the musicians were left to focus on their artistic vision, unperturbed by technical considerations, and engineers employed their scientific savvy to faithfully capture & frame the musicians' sound-world.
Obviously, much of the best music is a synthesis songwriting and production, but it's almost never the result of vague intuition or ham-fisted fuckin' around. Those leading lights equally famed for their musical and technical prowess - Quincy Jones, Brian Eno, Steve Albini - began as musicians and gradually developed their own production styles by being inquisitive, assimilating experience, and spending a lot of time in studios. It's precisely because of their intellectual curiosity that polymaths not only craft some of the most interesting music, but are more engaging in discussion. Intellectual laziness breeds artistic laziness, which in turn spawns boring albums and bad interviews.