Yeah thats an issue. You can't force them to do it but I think some new game mechanics might make people more drawn to them. More fun to use turrets or some design enhancement. Maybe a larger bunker for indoor combat where a spawn point is? Soft points inside make planetmans want to go in and defensive turrets make them want to deal with it.  


WAIT... Inside the giant bunker is a power generator overload that when overloads, completely nukes the base! You know damn well no planetman can resist triggering that.