One thing is not to be an asshole and respect people, but other is that janitors seem to not understand that when people who sit in the office after hours probably are doing unpaid overtime (in some companies you can at least take those hours back) and would prefer to finish their tasks as fast as possible and go home.
I used to work 9 to 5 (sometimes 9 to 8) and sure, I can greet the janitor, have some small talk, but I dont have the time to listen to their life story. And some janitors really liked to talk a lot, while I just wanted to finish my job and go home. (If they started at 6pm they were fresh at the beginning of their shift, while I should have already left 1 hour earlier).
So I understand why my colleagues (especially ones who had kids) ignored the janitors. If you leave 15 minutes earlier, you have 15 more minutes for your kid. If you are late the kid will be asleep.
And yes I talked with janitors, but I remember that one lady who thought that everyone can talk with her for 30 minutes at 7pm and just wouldnt take a clue to move on.
She didnt understand that the team was stuck at work after hours to finish something important, it was an abnormal situation.