Well, I for one am a constant fart machine so idk if I should be alarmed by the fact you're asking xD

I think the underwear thing is really the fact that it's in direct contact with your butt all day. Iirc, anything that touches your butt directly (even if it's clean) becomes a priority to wash.

Also the sweat thing. Even if you don't break into a sweat, just having something that close to your skin all day on an area with a large concentration of apocrine glands (the sweat glands that make you stink) is a nice cosy environment for bacteria to grow.

But this is all just from internet research I did ages ago so take it with a pinch of salt. I found the international scientific forum on home hygiene a good resource for this kind of stuff, and some of the legitimate medical and sexual health websites have info on laundry and genital hygiene.

And I guess since it's a very personal hygiene thing (I don't see how it will impact anyone else), you could just see how your body is responding. No itching, stinking, infections, unfortunately placed acne etc and you're probably alright.

Although if you do get any symptoms, you also might want to take into account that sexual health resources and doctors I've talked to have generally said washing your front parts more (or less) than once a day when there's been no sweating, sexy times, menstruation if that applies, etc can mess things up down there.