They aren’t!

Two things:

1: Americans are not actually very litigious relative to other countries, what you’re referring to is a culture projected by corporate interests violating the rights of individual Americans, interests invested in telling American citizens that they are too litigious, resulting in citizens who will therefore abstain from legally defending their rights when those rights are violated.

2: legal advertising became legal less than 50 years ago in the United States, because it’s obviously unethical and societally harmful. at this point, legal advertising is basically unregulated in the US.

Because The US allows legal commercials and advertisements on billboards and very importantly, American culture is the salient exported culture globally, lawsuits seem wider spread in the US and US culture than they actually are.