It’s honestly just not the case that the couple can afford everything.
When a baby comes, overnight you need like 100 things. My partner makes great money, but there’s now way we make enough to suddenly have a crib, a dresser, a changing table, a car seat, a stroller, lots of clothes, bottles, a baby bath, baby wash cloth, a ship full of diapers, baby wipes, bedtime story books, a bookcase for them, baby spoons, baby shampoo, a baby toiletries, tether, toys, the list goes on and on. For a second baby, a baby sprinkle, it’s more about squirting diapers and wipes, a 2nd car seat for the baby, a stroller than can hold two, things you need that you you can’t just reuse from the first baby, etc.
Plus it’s also a way for people to feel involved. My gave me baby clothes that I worse at my shower, my sister in law gave me new bibs that she found works better than others, my sister handmade me a baby blanket that is sentimental, etc.
Babies are very expensive, no matter how rich you are. Baby showers are a way of supportive group funding.