The U.S. federal government sets a minimum number of flight attendants on an aircraft for safety (generally, one per 50 passengers). But while many flight attendants announce that they are there “primarily for your safety” (don’t expect more than that!) they’re also the people in most direct contact with customers delivering an airline’s service.
So-called ‘full service’ airlines have historically staffed flights with more than the legal minimum. That started changing even before the pandemic, but accelerated when they weren’t providing much service at all.