In big cities like NYC people get sunlight during their commute, but get no sunlight when cooped up in a small apartment. Vitamin D deficiency has been long proven to inhibit proper functioning of the immune system, but this isn't widely known which is a huge failure of the press and the authorities.

    In big cities like NYC people get sunlight during 
    their commute

Perhaps you have only seen NYC in movies. You can spend a lot of time commuting in NYC without any direct sunlight hitting your skin whatsoever.

Not sure when you think they're soaking up all this sunlight. Sitting in a train car? ...that's underground? Or inside a bus? Or is it when they are walking between tall buildings?

New Yorkers are not exactly known for their tans, even in summertime.

    but get no sunlight when cooped up in a small apartment

You were not forced to stay in your apartment, even in NYC. You were always allowed to leave. Of course, most businesses are/were closed, but you are/were certainly allowed to walk around. Many people also have access to their apartment rooftops.

Perhaps more relevantly, of course, most Americans do not live in places like NYC where they have difficulty accessing sources of sunlight. I live in a suburb of a different American city and we are getting much more sunlight during quarantine. It's much easier to take walks and sunlight breaks during the daytime.

it's also specifically very good for respiratory diseases apparently.

That's definitely my personal case.

I moved from Brazil to the UK 6 years ago and have since developed a semi-transparent quality on my skin.

1 month into the quarantine and I have a great tan and look like a healthy human. The healthy part is probably in part due to better sleep and a lot less exposure to pollution but sunlight definitely helps.