not entirely sure what medium you're referring to, but chances are it's the quietest place the person has access to. Or it could be a scheduling matter.
I haven't noticed so much any interviews involving a questioner and a person being questioned. As for such things as the social media methods, people can control their environment better in their car rather than elsewhere, but most of those are just commenting on current events.
I don't watch a whole lot of that stuff but talking to somebody in their car, Jay Leon comes to mind.
He always asks the person questions about their car and that makes it interesting.
It gives the viewer a sense of direction with plot and such, added that it is less maintenance than say a live set, cheaper costs involved and I would imagine if done with go pros or similar easy to edit.