The other part that happens is that it becomes a time to discuss rumors, gossip, and dirt that I had no interest in or need to know or in a few instances created a toxic workplace with certain clicks. People who have had bad experiences will tend to avoid having lunch preferring their own company. The workplace has had potlucks and other events during lunch that on those occasions I would participate but aside from those events, I valued my downtime to get ready for the afternoon. It’s not perfect and people have found where I’m hiding to have my lunch to ask questions that frankly could have waited until I returned from lunch.
Not everyone wants to have lunch with others, recognizing your own need for self-care. I wouldn’t ask them why. as much it would be interesting to ask yourself why this is important to you. There might be many reasons why they chose not to have lunch with others. Just respect their wishes. Just respect that lunchtime even 30 minutes is not a long time but it may make a difference in how they end the day for themselves and you. I know my limits and to avoid burnout I prefer lunchtime alone.