In many offices, the majority of the employees start at about 10 am or even later. Also, most people people prefer to go home using public transportation, which stops relatively early (in most places, between midnight and 1:00 am). Extroverts like to socialize, introverts less so. Personally, I fall into the latter camp, but I usually find it to be invigorating and educational when I do "participate." Those with families often "bow out" (no pun intended), although I can see that with some groups, there might be peer pressure. If one has a long commute, that can be a perfect excuse to not participate or to leave early.