Jets and Giants are in NY, the worst hit place in the country
Other teams are not (the Bills are basically freaking Canada)
Most NFL teams are in big cities. Most of the country is getting this virus. New York is one of many cities getting hit, even if it's getting hit the worst.
Plus, the Jets and Giants aren't in New York. They're in New Jersey. It's not like the players are trapped in the city. They don't even have to go to the city!
Also, the Jets signed a free agent to a 1-year deal like 2.5 hours later.