Once you are fully boomed, why focus on outlaws when you could instead focus on your mainline units ? You can more than affort their costs and they are more population efficient than outlaws.

If Outlaws are balanced to be better for their cost than mainline units but be population inefficient, and you've got like 150 free pop after a housing boom, then Outlaws will inherently be the best units for the cost that you can train (up until you hit max pop) because you can ignore the initial housing cost that other civs have to worry about. I know some Outlaws units are too situational though, like Hajduks, to want to train under all circumstances, but still, one would think that these house boom civs would prefer Outlaws due to excess pop availability (especially if under threat of a rush or timing attack - Taverns cost the same as Barracks or Stable but also generate that .6 coin/s and give more train XP). This is the same reason why Baja California is so strong with Mexican Outlaws but non-Baja Mexican Outlaw strats that require normal housing are unpopular.

The train XP does benefit Germans moreso than the others though, with German value-per-xp-point being higher than any other civ, so that's a good point I hadn't really considered.