To be fair, you don't know if that couple is just out for dinner because they didn't make anything at home that day. You don't know if they've been around eachother and off their phones for the past 24 hours. I have the opposite anecdotal experience to yours. Lots of people at restaurants are not on their phones but I've also stopped caring to look honestly.

Or maybe one of them is responding to another human being in their life over their phone, and as that results in a break in conversation, the other person starts using theirs.