This may be because they are too small to have enough leverage to burrow, or because they are too large to make the effort worthwhile, or in some cases because they move relatively fast (for a millipede) and are active predators.
Isto pode deberse a que son demasiado pequenos para facer panca e soterrarse ou porque son demasiado grandes para que o esforzo lles pague a pena, ou nalgúns casos porque se moven relativamente rápido (para seren milpés) e son predadores activos.