Beyond that, a quirk of the American system in many states is registering your party affiliation at the time you register votes so that you can participate in primaries. When you've got >45% of registered voters identifying as being a supporter of one of the major two parties closely enough to add it to their name on the electoral roll, it's not exactly an encouraging environment to break that status quo

Other majoritarian democratic systems often also converge into two party (or two and a bit party, or two parties per region systems) but few seem to normalise voting for the same party every time in quite the same way.