You're probably oversimplifying the study considerably, but even so, even things that "everyone knows" need to be formally studied. All across science, non-intuitive results happen all the time. In psychology, it's even more common, since biology doesn't play by the relatively clean rules of materials physics.

After all, "everyone knows" that blacks are inferior and only useful as slaves (they even want to be, deep down); "everyone knows" that women are too temperamental to vote sensibly; "everyone knows" that people of that religion over there eat babies and we should destroy them before they destroy us...

For a more recent example, "everyone knows" that young people use condoms more often now, given the higher levels of sex education about pregnancy and STIs they've grown up with - yet regional studies often show significantly decreased levels of condom use. Another one is the assumption that the current crop of young'uns are fantastic with understanding computers due to growing up with them, yet this hasn't been borne out in studies. Turned out that consuming from a device doesn't mean you understand how it works any better.

Such studies are particularly important at ferretting out what's happening with people who aren't society's favourites - we all know what a man is, right? Always looking to get laid, not afraid to get physical, plays sport, drinks beer. Most men are like that, right? Not really; there's actually a huge variety of interests and desires. Studies of 'obvious!' things are just as necessary as fringe things, because sometimes the results are really quite unexpected.