Some are "welfairies", some are Catholics, some just like having babies, and some have lots of "accidents".
At any rate, you have to have a fair few kids to "make a profit" out of the state benefits. You'll also find that you can't keep having them forever, and when you stop the cost will really start to bite.
Historically, back a few generations, there were frequently up to 15 kids in a family in my family tree. Doesn't bother me, as long as all the kids are brought up well.