My serious guess is the knights or any other 2nd ML don't have enough power/status/money to protect the FL, like when the FL needs a powerful authority to vouch for them or be their backer for their "extraordinary" ideas that people might oppose. It's just easier to have the ML be the most powerful man so he has more plot uses. It's honestly tiring, because they make the 2nd ML the GREENEST FLAG not just in looks, sometimes lol, but PERSONALITY, and strength, only to add him as some obstacle the ML needs to help him realize his feelings/mistakes toward the FL or for the FL to realize her feelings for ML. Like usually, the 2nd ML is perfect except for his status (i.e., not enough money or power), and it's so annoying. Especially when the actual ML lacks so much in comparison, the dude would be enemy number one if he wasn't handsome. Sorry for the rant, lol.
Reminds me of that one ad in which all the princesses make fun about how people think all the girls worries should disappear when a strong tall prince show up
in all chinese/korean/japanese manga/manhwas, the FL will always end up with the best love interest. It's never gonna change. Now they can be the best in terms of looks, power, money, academics, etc. but the best one always wins.
But why always the emperor????like I think the knight also likes her(jin/jen) why it cannot be a knight/commoners like I know emperor kind n handsome n attractive but the knight is also kinda handsome n cute n is a good hubby material ヽ(`Д´)ﾉ