the most politically polarized are more actively involved in politics, amplifying the voices that are the least willing to see the parties meet each other halfway

The problem is that these hyper-polarized people often have very non-mainstreem views about a few issues. This causes the mainstream/moderates leave the party because they can't identify and don't want to associate with people they see as crazy or extremist.

The definitions of liberal and conservative do change over time... In the 1920s, I would have been around liberal. Today, I am a strong conservative.