The answer to this is the same answer to everything: 42. Kidding! Sex. It's sex. And Evolution (take that, creationists!). The ability to play music, and play it in a certain way, is something that's been selected for as an evolutionary advantage.
What follows are the science-based Top 5 Reasons Why You're Attracted to Guys (and Gals) in Bands, with visual aids explaining how we can see it in action right here in L.A.--kind of like a Discovery Channel special come to life:
5. Making music shows who you are as an individual.
Local Example: Wild Pack of Canaries. Because the people in this band are definitely individuals.
This is a corollary to the "cultural courtship model", which we'll go into later, and basically, the idea here is that musicianship is a vehicle for self-expression in a setting that best allows others to appraise our physical, mental, and emotional health--when we're on stage. It is not easy to stand out from the crowd, and making music--potentially a self-expressive endeavor that is costly in terms of time and energy with no clear survival benefits--can allow a venue for showing off creativity, mental health, and physical health in a context that lends itself to spectatorship. No one will want to reproduce with you if they don't even know you exist, and making music gives you a chance to stand out from the rest of the crowd in some respect.
4. Playing music in a group assists in the formation of social bonds and in putting forth the perception of a strong coalition.
Local Example: Jail Weddings. Because this is one strong coalition of mofos with cemented social bonds.
This is known as the "social cohesion hypothesis", and basically, the idea here is that making music together helps cements social bonds, and signal to other groups the potential for meaningful cooperation among members of a given group. In this model, the perception of music quality correlates with perceptions of coalition quality --meaning, in other words, that a group that plays music together, in time, without screwing up, is one that is perceived as a coalition of people that you probably should not fuck with.
3. Playing music shows that you are smart enough to remember things, and have some physical coordination and dexterity.
Local Example: the Entrance Band. Because these players have serious coordination and dexterity! [plus, Paz is mad hot with or without a bass, but especially with a bass!]
So far, the studies supporting this model have primarily been done on frogs and birds. The jury's still out insofar as frogs are concerns, but lady birds (female avians, not the black-and-red bug) love lots of mating songs, as evidenced by the fact that boy birds with bigger repertoires have more lady bird friends. Birds with small repertoires, not so much. And so birds with less vocal abilities lose the sexual selection game. This model seems to carry some weight in humans, if you buy into the mate choice (versus aggressive rivalry) sexual selection theory, since music seems to involve specialized memory capacity .
2. Playing complicated music shows that you are creative
Local Example: Dibia$e. Because Machines Hate Me, released last year on Alpha Pup, was insanely complex and chopped up some incredibly creative samples!
This model is predicated on the idea that cultural displays like art and music evolved due to sexually selected indicators of creativity, which also explains why so many forms of artistic expression are ritualized: comparison between courtship rituals, and by extension the genetic fitness of a given set of musicians, is easier when the displays have a lot of elements in common. For example, having set genres like "punk" and "rock" and "electronic" make it easy to tell who is good at punk and rock and who isn't. Having clear standards for "good" guitar playing makes it easy to tell who is "bad" at guitar playing. This could be why people are so resistant to absract art and atonal music--there's no clear and universally recognized form, making it hard to tell if composers or performers or even any good at what they're subjecting us to. This is called the "cultural courtship" model.
And the Number One Reason Why You're Attracted to Guys (and Gals) in Bands is... (here's the mindblowing punchline):