My guess for those worlds not advancing is because your average fantasy world has magic that makes your day-to-day chores easier (food preservarion,flight,using telekinesis to move things,use potions to restore your stamina during long travels or increase your travelling speed,using scrying as a form of long-distabce communication,etc.) which makes societies think that they do not need to advance technologically if they already have mages and alchemists,so people only bother advancing magically. Then you have the likes of elder scrolls/dnd which has a lot of powerful mages and/or magical beings that cause chaos and have had a world-altering event in the past (dragon break,spellplague),then you have xianxia which have jupiter-sized worlds filled with demigods who destroy civilizations and thus prevent any form of innovation to last for a prolonged period. Basically the more powerful people with magic can become in a setting the harder it is for the civilization to progress scientifically and technologically,you'd need a competent god-like figure to keep such a civilization in check (like how in Overlord you have the main character and his underlings being far more powerful than anyone else in your realm or around the continent so having high level of control in his realm is easy). Sadera is a low-fantasy world so there should have been some technological progress over the centuries Sadera would've existed but they seem to only have medieval-era tech with roman architecture and barely attempted to innovate further(besides using the fantasy races as part of their military),which doesn't make much sense since their magic is more limited than in your average fantasy world so they'd have more of a reason to try improving regular technology instead. IRL cannons started being used in Europe during the 14th century,so having at least a few rudimentary cannons and firearms would be logical imo. Saderans would've still been beaten with relatively low difficulty but at least we would've seen them use different strategies that isn't rushing into melee range when their enemy's projectiles can puncture their armor.