re: Why have the last 20 years been less culturally distinct than previous decades?

I'd take a different angle:

Mass storage and consumption mediums.

Let's put it this way:

When you were a kid, if you wanted to listen to your paren't music, you'd need to have a working phonograph, or buy limited supplies of relatively expensive media with shelf-life replayability.

Now, you can purchase/access/consume almost any media for a lifetime, with nearly unlimited selection, for subscription level fees.

That means a greater and wider instantaneous demand for entertainment in any form can be met with greater speed and diversity than ever before.

Americans didn't stop creating culture. They just started having access to all the aspects of created culture that had previously been only accessible in niche communities, or with specialized attention.