It’s not perceived as sad because it’s just something most people have no desire to do. Flags just aren’t a super common decoration you see outside of store advertisements and official government buildings. “I should install a flag pole on my property” is already a rare thought in most places and a lot of people then rather put a flag about something that is special to them on there. In line with that thought, being German in Germany obviously isn’t that special, so it’s usually not your choice of “displaying something that is special to me” unless you have a right-wing mindset. You’ll more often see football teams, maybe music bands and the more rare political issue here and there, like “stop nuclear power plants”.