I think it stems from 3 main factors:
1. Nearly nobody likes the smell of poop, you could argue it is our olfactory sense interpreting that smell and identifying it as bad but it is effective in deterring people from wanting to be near it. Even changing small babies that are messy can be an unpleasant chore and most parents/caregivers won’t hesitate but yes they will often grimace.
After 2-3 years of ever changing smells, frequency, consistency, increasing child size/weight and dealing with blow outs most parents/caregivers are tired of it and that sense pervades society.
2. Part of the getting children out of diapers strategy by lots of parents/caregivers is to belittle wearing diapers. It can be an effective strategy for sure and most kids will take on what their parents say and believe it as gospel.
Sure kids can be cruel but really it’s just peer dominance and asserting themselves, coupled with belief that diapers are not appropriate lots of kids out of diapers will believe they have achieved a dominant act and kids still dependent may feel shame... That doesn’t go away as people get older, it often becomes an unconscious core belief.
3. Dealing with infirmity is not something easily done. I am not saying people don’t have compassion but it does take a very very special person to be a caregiver to someone that isn’t a baby.
Sad to say that most of us recoil from what we consider sickness/infirmity it challenges our own sense of mortality, for younger adults it’s even worse with those selfish hormones running rampant lol.
I do think as a society we are slowly changing (lol the pun), part of it from greater understanding diaper wearers are not necessarily disease spreaders and part of it ABDL not fully in the shadows.
Putting it altogether it reminds me of people’s belief in Santa when they are young and then the manic belief they need to pass that belief onto the next generation when they are older.