It's not just the US. This happened across countries in Europe as well; Italy had it happen, Germany had "Coronavirus Parties", even. It happens because some people are idiots, and others aren't quite aware of how serious the situation is.

Once reality sets in, only the absolute idiots that don't care for being socially ostracized (or in some cases even legal consequences) will continue that behaviour. And luckily those are normally a minority.