Because we were founded on the idea that you can do whatever you want so long as you’re not imposing yourself on others. Things definitely have changed though. Yes, they did change. Now black people can do whatever they want so long as they're not imposing themselves on others, too. Agreed, same for women. Wasn’t perfect when we were founded, still isn’t perfect now. But the fact that these principles can be translated into modern culture is a testament to how universally true these rights are.

I'm not liberal and wouldn't really support the Civil Rights Movement, integration etc. if I lived in the 1950s-1960s, but it's true they're more free now than they were back then. It's a question whether that had a good impact on the rest of us, but it's the case.