They think they're such good drivers that they'll never get in accidents, so they wear the same underwear several times between launderings, because they think no one else will know. All men are immune to their own smell, from a distance longer than the length of their nose. Most men are physically unable to position their nose close enough to their butt to smell it, and those who can, don't. They already know what their butt smells like, from when they did that as a kid. A guy's butt is going to smell the same way as it did, unless he's eating different food.
Women don't drive well enough to risk wearing the same underwear two days in a row. That's why they take cabs or public transport if they sleep over on a work night.