Same. And until i had covid, i had a really excellent sense of smell.
When there’s a poop odor, it’s usually due to a plumbing problem. Not due to “build up”. And a urine odor is almost always from men who don’t aim well.
Ahhh, the people who think their shit doesn’t stink. 
I dunno about you, but my shit stinks a LOT more when I’m sick than when I’m healthy. And it depends what I’ve been eating, too. Fish makes my poop more odorous, for instance. Old poop smells horrible (yes, I’ve used a lot of outhouses) but fresh poop from a healthy person who has mostly been eating non-allium vegetable matter smells like poop, but isn’t really that bad.
Though the worst outhouse I’ve ever smelled was years ago on, I think, the Pennsylvania Turnpike; where they’d tried to cover up the smell with some sort of scented cleaning product. The combined stink was I think tied for the worst thing I’ve ever smelled in my life. Much worse than any untreated outhouse, and yes I’ve been in quite a few also.
The purpose of the fan is to keep the stink from escaping the bathroom until it’s dissipated. The fan ought to stay on for 15 mins or so, with the door closed or slightly ajar. I keep meaning to set up some sort of timer on mine so the kids aren’t leaving it running endlessly.
A lot of that smell is rancid urine. I used to work at a rafting outpost that had an outhouse, and the etiquette was that you don’t pee in the outhouse, you find a secluded spot in the woodline somewhere. It makes a night-and-day difference between an outhouse that’s comfortable to use vs. one where you need to hold your breath, finish your business as rapidly as possible, and burn your clothes afterwards.
Yeah, decades ago, there was an outhouse at Consolation Lake (on the Mt. Whitney trail) which was for poop only. You were supposed to pee elsewhere.
Anyway, even a moderately clean bathroom with functioning modern plumbing doesn’t smell of stale poop or stale pee, unless someone isn’t using it properly. You don’t need to run the fan just for that. You need the fan to prevent mildew after showering, and it’s handy to remove odoriferous farts, too.