One More Thing I Don’t Understand

We went out for lunch today at a quick casual restaurant. There were five of us. When we were finished ordering and wanted to pick a table, we had a difficult time. There were a lot of tables and booths available. They were all for three or four people. Every single five or six person table or booth was taken. All but one of those was taken by only one or two people. One was being used by a party of four. What makes a single person think that they should take up a six person table? It just would not occur to me.