1. There are lots of unproven claims made for milk thistle by advocates of herbal medicine.
2. The US has an awful lot of people who will buy pretty much anything if they are told, by word of mouth, that the vitamin / supplement/remedy will improve their health. Not much different to here, really.
3. You only have their word that "7 out of 10" Americans use it. Seems extremely unlikely to me.
4. There are some trials which suggest that milk thistle may have benefit in those situations where there is liver damage - but, that evidence comes from poorly designed trials. The better quality trials seem to show no benefit at all.More trials, and more data is required before this remedy is shown to be efficacious.
You have to take claims of the benefits of herbal remedies with a pinch of salt. People just love the idea of a "natural" remedy, and can often fool themselves as to the benefits they are seeing. As for the manufacturers of such remedies, well, it benefits their pocket if they can persuade more people of the frankly dubious health benefits of many of their concoctions.