Because there's no PR, anyone who wants to be effective has to join one of the parties. Because of that, diversity in each party is way higher than you see in places that use party list PR.

For example, the Democrats have both Alrxandria Ocasio Cortez, who describes herself as a democratic socialist, and Joe Manchin, a fairly conservative former coal exec from West Virginia.

The real interesting bit of American elections tends to be the primaries, where all the voters registered with a party vote on who their candidate should be. Alexandria Ocasio Cortez got elected because she defeated the incumbent Democrat in the primary.

In 2020, everyone from Bernie Sanders to billionaire Michael Bloomberg ran in the Democratic primary. There was some real choice there until Biden won and it became just Trump vs Biden.