Back when I was born it was the done thing in hospital before coming home. Unless you opted out. Pretty much all males around my age and older would have had it done.
By the time I had my kids it wasn't the norm unless you opted in and only private hospitals would do it. So my boys aren't done and neither are most boys their age. There is a small percentage who have done it by choice and have had to arrange that which is costly.
So the SATC ladies being older than I am, for their generation it would have been the done thing when they were born that boys had it done in hospital right after birth. Which is why that would have been in the show.
I personally don't think it should ever be done unless medically necessary but I can see why it was a thing in SATC being generational. Don't know how prevalent it is in the US now, but where I am they no longer do it.