I think one reason for this is that the base variant is easy to understand and there's no 'map' to learn making it a great fit for casual, pick-up play. BZFlag is the OG Q3DM17.
At least you said Longest Yard and not Campgrounds.
In my first full-time programming job (only back in 2008!), we used to play BZFlag at lunch. It was the only fun thing you could install through the CentOS package manager. A year or two later, we had Quake 3, with Lego mods.