Not covered in first answer: you could leave at 6, but if you stay till 7 there's a roast dinner. I suspect people who still work at Google are sometimes a little blind to their own behaviour.
I bet they even make excuses before leaving at 8 pm... Its really very sad that smart human beings act like that due purely to social pressure.
This is true for the 3 big tech companies I've worked for (which is the topic of this thread). Sometimes I get in at 10, sometimes 7. Sometimes I leave at 4, sometimes 8. I've yet to have someone comment on my schedule.
Yet I continue to see people say stuff like "oh free dinner is a trap! ahh!" "A gym on campus? Oh don't fall for it!" I don't get it.
Edit: it's not free but highly subsidized.