It is weird. I think it is in part because America is the first enlightenment nation. We have no blood or deep history binding us together, so superficial and melodramatic shows of solidarity become much more important in generating a stable sense social identity. Think of it as a bunch of deracinated individuals attempting to say "see, 'we' are a 'we'!"

The puritans were virtue-signallers par excellance, so that probably also flows through to what you see today.