The expectation is that the very young, and the elderly, can't control it. Everyone in between would only use a diaper because they're disabled, lazy, or have a fetish.
People forget that not all disability is obvious or visible, and even "normal" bladder/bowel control isn't uniform across all people. It's also a cultural side effect of tactics used in potty training. "Diapers are for babies. Big boys/girls use the potty". Then shame and guilt to sure up the gap. It's a terrible tactic to begin with, but then some people can't compartmentalize, and apply the same twisted logic to adults.