Part of it is that more complicated interactions tend to create instabilities: https://physics.stackexchange.com/questions/384910/can-einstein-hilbert-action-be-derived-from-symmetry-considerations/385024#385024
Part of it is that more complicated interactions tend to create instabilities: https://physics.stackexchange.com/questions/384910/can-einstein-hilbert-action-be-derived-from-symmetry-considerations/385024#385024