@memeSlayer09 At a guess, judging from Frieren saying it was tiring, it's super hard for not a whole lot of gain. Well there is GAIN, since it seems that training in that state does lead to improvement in some way, but it's not like...worth it?
Like it's probably a thing for training purposes for sure, it's learning to control a part of yourself better. But doing it constantly? All the time? Forever? Why bother with the exception of fucking over people who can sense that sort of thing which seems to actually be a hard to develop skill? So far the only people we've seen who're able to do it are high end mage types and or demons and even they can't tell at a glance.
If anything as it's being used, it's not really a
mage thing so much as a
demon slayer thing.