There’s also a matter of size. Babies come in pretty standard sizes overall (ranging from tiny preemie to tiny newborn to toddler, and then a bit of extra market for in-between ages), but adults…
What works for one country does not work for another. I, for example, cannot buy underpants made for the Chinese market (I’ve tried) because the dimensions are just wrong. Even when scaled, they don’t fit right. The body proportions are just different for adults from different places, and that’s ok, but it does make the adult diaper scene similarly localized.
After all, you can make a one-size adult diaper, hospitals use them, but they have strong limitations for how hideable they are (most adult diaper products are marketed to women who suffer far greater rates of incontinence due to childbirth, so hideability is important).