The human mind is remarkably good at filtering out constant low-level stimuli/noise. Consider, for example, how hard it is to smell what your house smells like. I'm not talking about unusual odors, like a recent growth of mold, or strong smells like an open trash can or cleaning solution. If you've been to other peoples' houses, you know that every dwelling has a scent... except yours. Because you don't notice it. You subconsciously filter it out. (If you really want to know what your own house smells like, go away for a weekend, or a week, and pay attention when you first open the door - before your brain has a chance to adapt to "the new normal" and edit it from your perceptions again.)

Likewise, people who live in noisy environments learn to ignore the noise, and may even feel disturbed (consciously or not) without it. Experiments were done with glasses fitted with prisms that inverted the view of subjects; their brains eventually adapted to the flipped visual input and they were able to function normally, until the glasses were removed and they had to learn how to see the "right" way all over again.

So that's the "how do you stand it" part answered - they honestly don't notice. As to why?

Hygiene and personal grooming takes time and effort, even for males (qualifier added because, in my experience, the level of "acceptable/minimal" required maintenance is much higher in this society for females). If you're feeling lazy, and you don't think anyone's going to notice (because YOU don't notice, see above), and/or you just don't care or think such things are important (along with fashion and social graces), you might be tempted to skip bathing every other day, or not brush your teeth EVERY morning, or whatever.

I wish I could say I don't speak from personal experience here, but we all have things in our pasts we regret, yes?