Firstly the link to ERIC above is useful for you to read.
We produce a hormone called vasopressin which suppresses urine production at night, no full bladder, no need to empty it.
Ds wet the bed until he was 10. Because of school residential trips etc we took him to the GP at 7. He was prescribed Desmopressin which is the synthetic hormone of vasopressin.
It worked, instantly dry. However, we were not prepared the medicate him all the time so we just used the Desmopressin for school trips, sleep overs and holidays. We even monitored Ds's input and output, ie he had a wee in a measuring jug so we could monitor everything.
They cannot help it if they do not make the hormone, that comes with age and it can be late. Firstly I still think there is a stigma attached to bed wetting, but they are asleep, they cannot help it. Secondly, supermarket shelves are filled with pyjama pants, why do you think they would give over precious shelf space? Because lots of children are not dry at night for a long time.
Ds wore pull ups which massively helped. I also wet the bed as did my Dad.