It's actually extremely common for January in NJ to have temperatures in the 50s. I've lived here my whole life and it's been this regular occurrence I've observed for decades for people in this state to be like "It's in the 50s in January?! This weather is caaaaaa-razy!" despite this not being uncommon at all. Pick a year and look up daily temperatures for NJ in January and you won't have a hard time finding 50s.

People also don't seem to realize that in terms of world geography, NJ is actually relatively far south for those of us with European ancestry, like on the level of Spain, Southern Italy, etc., and get more direct sunlight than where many of our ancestors came from. This is important as SAD is largely associated with UV light exposure.

In general, the point isn't that we don't have cold stretches of days where maybe indoor facilities would be useful, but our winters are so short and speckled with stretches of warmer, sunnier weather, that that's a lot of wasted money and space most of the year.

Judging from your user name, you are probably used to colder winters than some other counties, so that might be a fair point for those counties!