“On a side note, why shouldn’t there be hechsherim on clothing for shatnez? Talleisin have hechshers, but I’ve never seen it on normal clothing. Does it exist, and I just don’t know about it?”
It exists for certain chassidish clothing. the problem is that most clothing is made in the far east. it’s very expensive to have hasgacha on that. food in general is made under tighter conditions for fear of health problems, clothing is made often in sweat shops.