Why do they sit in the dark?

I teach a lot of morning classes but even later in the day I will come in to a classroom and there will be students sitting there (sometimes several) in the dark.

It's happened for the last few years now, I ask why nobody turns on the lights and they just shrug.

My take is they don't need light to see their phones so they don't use it but it is strange (and sometimes startling) that they sit there in the dark. I can't remember this being a thing in the past, anybody else seeing this?