Well season ninjas are meant to be limited and reward for playin that season same for all festive ninjas for new year/chinese new year etc.

All others you listed are still in events just not every week, Masked Man/Minato/Anbu Itachi are here every treasure of sage event and few others. Mizuki is a bit rarer, think he was only in his first give away event, sakuras gift and monthly event but pretty sure you can get him eventually, i have almost 4 star and i never checked him till you mentioned just now, so his frags are droping somewhere lol.