Is there a reason for not training Nezuko in breathing? I understand that Nezuko gets stronger from just sleeping but the entire time Tanjiro is training to master his breathing alongside Zenitsu and Inosuke she just sleeps. Would it not have made more sense to train her to breathe as well and she could sleep during travel and daytime and do breathing training at night? Or is this not done due to just plot, not constraints? Are demons even able to learn breathing techinques?
Nezuko sleeps not because she has nothing else to do but because in such a way she restores her strength instead of eating flesh. She slept through Tanjiro's training year, while (probably, not shown really) he tried to wake her up.

From the slayers' point of view, they most likely have no idea how to train demons because they have a different physiology. (Upper Moon One seem to have mastered the moon breath before becoming a demon.) Also, pillars still may think it is not safe.