Do they really? I had no idea..And I never saw an Amish when I lived there -if you mean NYC...?

The Mennonites send the boys out only, not the girls. The young Mennonite girls had too much fun and tended to stay more than the boys did.
There is also some sexual selection going on. If you want to have kids, a Mennonite community is a much better place to raise them. You can support a family easier than in a city.