The idealistic take is that the United States is a country that maximizes freedom, which includes the freedom to enter into whatever contracts you wish. You aren't forced to take a job that requires you to waive your right to sue; you're choosing to do it. If it were too important, capitalism suggests that another employer would show up, offer you a little less pay and no arbitration clause, and everybody would work for that company instead.

In practice, this isn't how life works, but that's the theoretical reason. The real reason is that pro-corporate politicians hold power and corporations want arbitration agreements to be legal, largely because courts are more expensive, bad PR, and are less likely to work out in their favor.