The goddess isn't the highest being, she belongs to a divine race and she's basically sent to the mortal world by an even greater divine being, she's the only divine being shown so far, it's not clear how much other gods are involved with everything. Alion the kingdom she's in is the only kingdom with summoned heroes because of her, and the king is basically just her puppet on the surface/in politics. She kinda has some unrevealed personal (speculated) plans manipulating the world, has her own powerful followers in the form of a cult (one follower character is introduced and she's basically blackmailed by the goddess with hostages to be the goddess's follower), it appears that she usually recruits fairly strong people (non-summoned isekai'd people) with underhanded methods (including female lead). Basically most if not all of her actions are spun to look "kind" and "justice" but are actually indirectly threatening anyone including kings to force them to obey her (a kingdom has suffered consequences because of it), probably all the upper authorities are aware that she's not benevolent at all but they don't want to go against her head on (there's a king seems to be less cooperative, perhaps a future ally? IDK yet I haven't read to the point where more about that character is provided). No reason is given so far about why she doesn't just personally fight the demon race or take over the world, but everything explained by her has been really fishy anyway so maybe she's restricted or the demon race isn't the true villain whatsoever, an important kind of golden essence related to a type of monsters and the demon lord also hasn't been mentioned in the manga so far (it's mentioned briefly in the beginning in the novel, the goddess seems to be seeking it, demon lord has relations to it, a classmate seems to be somehow related to it because of the colour too mentioned by MC), the demon lord's identity and motivations aren't even explained too yet. These stuffs will probably be revealed further along MC's journey.
