well I wouldn't say the designers themselves are vain, but maybe the advertisers they hire are.
Either way, using the waxed hunky models is the safest way to go with for various reasons, especially for the mass market products like Calvin Klein and Hugo Boss (Side note: "Klein" is German for small).
Most Gay and Straight men will look at the hunk on the package and think/know he is hot...Us gays might think he is attractive, while our hetero brothers will know that this is what society deems attractive men to be, and subconsciously both us gays and straights will come to a conclusion (even though we're not actively thinking about it) that b/c the guy wearing the underwear is "sexy" if we buy and wear the underwear we will become sexy to... at least a little bit.
Also many times in department stores like Macy's and Dillard's, the mens department is right at the entrance from the mall and the men's underwear are on display right at the front for everyone to see. This way when (straight) women pass by trying to get to another department, the hunks on the underwear package might catch their eye, causing them to buy a pair for their beau.
I will say however that the "designer/gay pornstar" brand underwears like Andrew Christian are a tad bit too vain though
