Things have never declared themselves empty, nor have they declared
themselves form; and they have not declared themselves right, wrong,
defiled or pure. Nor is there a mind that binds and fetters people.
It is just because...
ZAZEN MEDITATION GUIDE - Chapter 05. Garments for Sitting
Garments for Sitting You will not be comfortable with any kind of tight, heavy or hard fabric clothing when you sit in zazen. Therefore, any loose fitting garment made of fine, soft fabrics will be better in sitting. When you do zazen, wearing garment is much better than wearing nothing.